[1/2] drm/i915: Introduce execlist_port_* accessors

Commit Message

Mika Kuoppala Oct. 19, 2017, 2:39 p.m. UTC

From: Mika Kuoppala <[e-mail]>

Instead of trusting that first available port is at index 0,
use accessor to hide this. This is a preparation for a
following patches where head can be at arbitrary location
in the port array.

v2: improved commit message, elsp_ready readability (Chris)
v3: s/execlist_port_index/execlist_port (Chris)
v4: rebase to new naming
v5: fix port_next indexing

Cc: Michał Winiarski <[e-mail]>
Cc: Joonas Lahtinen <[e-mail]>
Cc: Chris Wilson <[e-mail]>
Signed-off-by: Mika Kuoppala <[e-mail]>
---
 drivers/gpu/drm/i915/i915_gpu_error.c      |  6 ++-
 drivers/gpu/drm/i915/i915_guc_submission.c | 53 +++++++++++++++----------
 drivers/gpu/drm/i915/intel_engine_cs.c     |  2 +-
 drivers/gpu/drm/i915/intel_lrc.c           | 63 ++++++++++++++++++------------
 drivers/gpu/drm/i915/intel_ringbuffer.h    | 38 ++++++++++++++++++
 5 files changed, 114 insertions(+), 48 deletions(-)

Comments

Chris Wilson Oct. 19, 2017, 2:48 p.m. UTC | #1

Quoting Mika Kuoppala (2017-10-19 15:39:41)
>         while (test_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted)) {
> +               struct execlist_port *port;
> +
>                 /* The HWSP contains a (cacheable) mirror of the CSB */
>                 const u32 *buf =
>                         &engine->status_page.page_addr[I915_HWS_CSB_BUF0_INDEX];
> @@ -855,7 +865,7 @@ static void intel_lrc_irq_handler(unsigned long data)
>  
>                         if (status & GEN8_CTX_STATUS_ACTIVE_IDLE &&
>                             buf[2*head + 1] == PREEMPT_ID) {
> -                               execlist_cancel_port_requests(execlists);
> +                               execlists_cancel_port_requests(execlists);
>  
>                                 spin_lock_irq(&engine->timeline->lock);
>                                 unwind_incomplete_requests(engine);
> @@ -870,6 +880,8 @@ static void intel_lrc_irq_handler(unsigned long data)
>                             execlists->preempt)
>                                 continue;
>  
> +                       port = execlists_port_head(execlists);
> +
>                         /* Check the context/desc id for this event matches */
>                         GEM_DEBUG_BUG_ON(buf[2 * head + 1] != port->context_id);
>  
> @@ -890,7 +902,7 @@ static void intel_lrc_irq_handler(unsigned long data)
>                         }
>  
>                         /* After the final element, the hw should be idle */
> -                       GEM_BUG_ON(port_count(port) == 0 &&
> +                       GEM_BUG_ON(port_count(execlists_port_head(execlists)) == 0 &&
>                                    !(status & GEN8_CTX_STATUS_ACTIVE_IDLE));
>                 }

Can you try reworking this such that port is kept local without having
to go back to the struct on every loop? And then compare code
generation.
-Chris

Chris Wilson Oct. 19, 2017, 2:50 p.m. UTC | #2

Quoting Mika Kuoppala (2017-10-19 15:39:41)
> +#define __port_add(start, n, mask) (((start) + (n)) & (mask))
> +#define port_head_add(e, n) __port_add((e)->port_head, n, (e)->port_mask)
> +
> +/* Index starting from port_head */
> +static inline struct execlist_port *
> +execlists_port(struct intel_engine_execlists * const execlists,
> +              const unsigned int n)
> +{
> +       return &execlists->port[port_head_add(execlists, n)];

nth_port cf nth_page?
-Chris

Joonas Lahtinen Oct. 20, 2017, 10:34 a.m. UTC | #3

On Thu, 2017-10-19 at 17:39 +0300, Mika Kuoppala wrote:
> From: Mika Kuoppala <[e-mail]>
> 
> Instead of trusting that first available port is at index 0,
> use accessor to hide this. This is a preparation for a
> following patches where head can be at arbitrary location
> in the port array.
> 
> v2: improved commit message, elsp_ready readability (Chris)
> v3: s/execlist_port_index/execlist_port (Chris)
> v4: rebase to new naming
> v5: fix port_next indexing
> 
> Cc: Michał Winiarski <[e-mail]>
> Cc: Joonas Lahtinen <[e-mail]>
> Cc: Chris Wilson <[e-mail]>
> Signed-off-by: Mika Kuoppala <[e-mail]>

<SNIP>

> @@ -561,15 +563,20 @@ static void port_assign(struct execlist_port *port,
>  static void i915_guc_dequeue(struct intel_engine_cs *engine)
>  {
>  	struct intel_engine_execlists * const execlists = &engine->execlists;
> -	struct execlist_port *port = execlists->port;
> +	struct execlist_port *port;
>  	struct drm_i915_gem_request *last = NULL;
> -	const struct execlist_port * const last_port =
> -		&execlists->port[execlists->port_mask];
>  	bool submit = false;
>  	struct rb_node *rb;
>  
> -	if (port_isset(port))
> -		port++;
> +	port = execlists_port_head(execlists);
> +
> +	/*
> +	 * We don't coalesce into last submitted port with guc.
> +	 * Find first free port, this is safe as we dont dequeue without
> +	 * atleast last port free.

"at least" + "the"

<SNIP>

> @@ -557,6 +557,9 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>  	if (!rb)
>  		goto unlock;
>  
> +	port = execlists_port_head(execlists);
> +	last = port_request(port);
> +
>  	if (last) {
>  		/*
>  		 * Don't resubmit or switch until all outstanding
> @@ -564,7 +567,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>  		 * know the next preemption status we see corresponds
>  		 * to this ELSP update.
>  		 */
> -		if (port_count(&port[0]) > 1)
> +		if (port_count(port) > 1)
>  			goto unlock;
>  
>  		if (can_preempt(engine) &&
> @@ -598,7 +601,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>  			 * the driver is unable to keep up the supply of new
>  			 * work).
>  			 */
> -			if (port_count(&port[1]))
> +			if (port_count(execlists_port_next(execlists, port)))
>  				goto unlock;
>  
>  			/* WaIdleLiteRestore:bdw,skl
> @@ -634,7 +637,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>  				 * combine this request with the last, then we
>  				 * are done.
>  				 */
> -				if (port == last_port) {
> +				if (port == execlists_port_tail(execlists)) {
>  					__list_del_many(&p->requests,
>  							&rq->priotree.link);

Nothing to fix related to this patch, but I was sure next hunk was
going to escape my screen :) Maybe we need to cut the indents a bit.

> @@ -890,7 +902,7 @@ static void intel_lrc_irq_handler(unsigned long data)
>  			}
>  
>  			/* After the final element, the hw should be idle */
> -			GEM_BUG_ON(port_count(port) == 0 &&
> +			GEM_BUG_ON(port_count(execlists_port_head(execlists)) == 0 &&
>  				   !(status & GEN8_CTX_STATUS_ACTIVE_IDLE));

Why did you stop trusting port variable here?

Other than that, looks good to me.

Regards, Joonas

Mika Kuoppala Oct. 20, 2017, 11:12 a.m. UTC | #4

Joonas Lahtinen <[e-mail]> writes:

> On Thu, 2017-10-19 at 17:39 +0300, Mika Kuoppala wrote:
>> From: Mika Kuoppala <[e-mail]>
>> 
>> Instead of trusting that first available port is at index 0,
>> use accessor to hide this. This is a preparation for a
>> following patches where head can be at arbitrary location
>> in the port array.
>> 
>> v2: improved commit message, elsp_ready readability (Chris)
>> v3: s/execlist_port_index/execlist_port (Chris)
>> v4: rebase to new naming
>> v5: fix port_next indexing
>> 
>> Cc: Michał Winiarski <[e-mail]>
>> Cc: Joonas Lahtinen <[e-mail]>
>> Cc: Chris Wilson <[e-mail]>
>> Signed-off-by: Mika Kuoppala <[e-mail]>
>
> <SNIP>
>
>> @@ -561,15 +563,20 @@ static void port_assign(struct execlist_port *port,
>>  static void i915_guc_dequeue(struct intel_engine_cs *engine)
>>  {
>>  	struct intel_engine_execlists * const execlists = &engine->execlists;
>> -	struct execlist_port *port = execlists->port;
>> +	struct execlist_port *port;
>>  	struct drm_i915_gem_request *last = NULL;
>> -	const struct execlist_port * const last_port =
>> -		&execlists->port[execlists->port_mask];
>>  	bool submit = false;
>>  	struct rb_node *rb;
>>  
>> -	if (port_isset(port))
>> -		port++;
>> +	port = execlists_port_head(execlists);
>> +
>> +	/*
>> +	 * We don't coalesce into last submitted port with guc.
>> +	 * Find first free port, this is safe as we dont dequeue without
>> +	 * atleast last port free.
>
> "at least" + "the"
>
> <SNIP>
>
>> @@ -557,6 +557,9 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>>  	if (!rb)
>>  		goto unlock;
>>  
>> +	port = execlists_port_head(execlists);
>> +	last = port_request(port);
>> +
>>  	if (last) {
>>  		/*
>>  		 * Don't resubmit or switch until all outstanding
>> @@ -564,7 +567,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>>  		 * know the next preemption status we see corresponds
>>  		 * to this ELSP update.
>>  		 */
>> -		if (port_count(&port[0]) > 1)
>> +		if (port_count(port) > 1)
>>  			goto unlock;
>>  
>>  		if (can_preempt(engine) &&
>> @@ -598,7 +601,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>>  			 * the driver is unable to keep up the supply of new
>>  			 * work).
>>  			 */
>> -			if (port_count(&port[1]))
>> +			if (port_count(execlists_port_next(execlists, port)))
>>  				goto unlock;
>>  
>>  			/* WaIdleLiteRestore:bdw,skl
>> @@ -634,7 +637,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>>  				 * combine this request with the last, then we
>>  				 * are done.
>>  				 */
>> -				if (port == last_port) {
>> +				if (port == execlists_port_tail(execlists)) {
>>  					__list_del_many(&p->requests,
>>  							&rq->priotree.link);
>
> Nothing to fix related to this patch, but I was sure next hunk was
> going to escape my screen :) Maybe we need to cut the indents a bit.
>

I have noticed the same. But I didn't feel like attacking this loop
until everything is in place and working.

>> @@ -890,7 +902,7 @@ static void intel_lrc_irq_handler(unsigned long data)
>>  			}
>>  
>>  			/* After the final element, the hw should be idle */
>> -			GEM_BUG_ON(port_count(port) == 0 &&
>> +			GEM_BUG_ON(port_count(execlists_port_head(execlists)) == 0 &&
>>  				   !(status & GEN8_CTX_STATUS_ACTIVE_IDLE));
>
> Why did you stop trusting port variable here?
>

We did assing it pre loop before. Now we do it inside the loop. Also
I thought I made a favour for reader (and for the bug triager
as GEM_BUG_ON might soon show condition in dmesg) 
to note that it is always the first port count we assert
for idleness.

-Mika

Chris Wilson Oct. 20, 2017, 11:26 a.m. UTC | #5

Quoting Mika Kuoppala (2017-10-20 12:12:02)
> Joonas Lahtinen <[e-mail]> writes:
> >> @@ -890,7 +902,7 @@ static void intel_lrc_irq_handler(unsigned long data)
> >>                      }
> >>  
> >>                      /* After the final element, the hw should be idle */
> >> -                    GEM_BUG_ON(port_count(port) == 0 &&
> >> +                    GEM_BUG_ON(port_count(execlists_port_head(execlists)) == 0 &&
> >>                                 !(status & GEN8_CTX_STATUS_ACTIVE_IDLE));
> >
> > Why did you stop trusting port variable here?
> >
> 
> We did assing it pre loop before. Now we do it inside the loop. Also
> I thought I made a favour for reader (and for the bug triager
> as GEM_BUG_ON might soon show condition in dmesg) 
> to note that it is always the first port count we assert
> for idleness.

I would favour assert(port == el_port_head(el)); where appropriate.
The intent is that we comment upon the rationale of the GEM_BUG_ON() so
that we have an aide-memoire as to where to begin the hunt. For
triaging, we just need to be able to recognise the same BUG_ON as you
only need to dupe to the first one to catch up with the bug analysis.

We should err on making GEM_BUG_ON() fit the code better than a
potential bug report. We should be reading the code far more often...
-Chris

Mika Kuoppala Oct. 20, 2017, noon UTC | #6

Chris Wilson <[e-mail]> writes:

> Quoting Mika Kuoppala (2017-10-19 15:39:41)
>>         while (test_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted)) {
>> +               struct execlist_port *port;
>> +
>>                 /* The HWSP contains a (cacheable) mirror of the CSB */
>>                 const u32 *buf =
>>                         &engine->status_page.page_addr[I915_HWS_CSB_BUF0_INDEX];
>> @@ -855,7 +865,7 @@ static void intel_lrc_irq_handler(unsigned long data)
>>  
>>                         if (status & GEN8_CTX_STATUS_ACTIVE_IDLE &&
>>                             buf[2*head + 1] == PREEMPT_ID) {
>> -                               execlist_cancel_port_requests(execlists);
>> +                               execlists_cancel_port_requests(execlists);
>>  
>>                                 spin_lock_irq(&engine->timeline->lock);
>>                                 unwind_incomplete_requests(engine);
>> @@ -870,6 +880,8 @@ static void intel_lrc_irq_handler(unsigned long data)
>>                             execlists->preempt)
>>                                 continue;
>>  
>> +                       port = execlists_port_head(execlists);
>> +
>>                         /* Check the context/desc id for this event matches */
>>                         GEM_DEBUG_BUG_ON(buf[2 * head + 1] != port->context_id);
>>  
>> @@ -890,7 +902,7 @@ static void intel_lrc_irq_handler(unsigned long data)
>>                         }
>>  
>>                         /* After the final element, the hw should be idle */
>> -                       GEM_BUG_ON(port_count(port) == 0 &&
>> +                       GEM_BUG_ON(port_count(execlists_port_head(execlists)) == 0 &&
>>                                    !(status & GEN8_CTX_STATUS_ACTIVE_IDLE));
>>                 }
>
> Can you try reworking this such that port is kept local without having
> to go back to the struct on every loop? And then compare code
> generation.

New one is with the last_port introduce and set outside loop.

add/remove: 0/0 grow/shrink: 1/1 up/down: 10/-7 (3)
function                                     old     new   delta
intel_lrc_irq_handler                       1847    1857     +10
i915_guc_irq_handler                        1327    1320      -7

Without looking at the assembler, I would go with last_port
as a net win.

-Mika

Mika Kuoppala Oct. 20, 2017, 12:53 p.m. UTC | #7

Mika Kuoppala <[e-mail]> writes:

> Joonas Lahtinen <[e-mail]> writes:
>
>> On Thu, 2017-10-19 at 17:39 +0300, Mika Kuoppala wrote:
>>> From: Mika Kuoppala <[e-mail]>
>>> 
>>> Instead of trusting that first available port is at index 0,
>>> use accessor to hide this. This is a preparation for a
>>> following patches where head can be at arbitrary location
>>> in the port array.
>>> 
>>> v2: improved commit message, elsp_ready readability (Chris)
>>> v3: s/execlist_port_index/execlist_port (Chris)
>>> v4: rebase to new naming
>>> v5: fix port_next indexing
>>> 
>>> Cc: Michał Winiarski <[e-mail]>
>>> Cc: Joonas Lahtinen <[e-mail]>
>>> Cc: Chris Wilson <[e-mail]>
>>> Signed-off-by: Mika Kuoppala <[e-mail]>
>>
>> <SNIP>
>>
>>> @@ -561,15 +563,20 @@ static void port_assign(struct execlist_port *port,
>>>  static void i915_guc_dequeue(struct intel_engine_cs *engine)
>>>  {
>>>  	struct intel_engine_execlists * const execlists = &engine->execlists;
>>> -	struct execlist_port *port = execlists->port;
>>> +	struct execlist_port *port;
>>>  	struct drm_i915_gem_request *last = NULL;
>>> -	const struct execlist_port * const last_port =
>>> -		&execlists->port[execlists->port_mask];
>>>  	bool submit = false;
>>>  	struct rb_node *rb;
>>>  
>>> -	if (port_isset(port))
>>> -		port++;
>>> +	port = execlists_port_head(execlists);
>>> +
>>> +	/*
>>> +	 * We don't coalesce into last submitted port with guc.
>>> +	 * Find first free port, this is safe as we dont dequeue without
>>> +	 * atleast last port free.
>>
>> "at least" + "the"
>>
>> <SNIP>
>>
>>> @@ -557,6 +557,9 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>>>  	if (!rb)
>>>  		goto unlock;
>>>  
>>> +	port = execlists_port_head(execlists);
>>> +	last = port_request(port);
>>> +
>>>  	if (last) {
>>>  		/*
>>>  		 * Don't resubmit or switch until all outstanding
>>> @@ -564,7 +567,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>>>  		 * know the next preemption status we see corresponds
>>>  		 * to this ELSP update.
>>>  		 */
>>> -		if (port_count(&port[0]) > 1)
>>> +		if (port_count(port) > 1)
>>>  			goto unlock;
>>>  
>>>  		if (can_preempt(engine) &&
>>> @@ -598,7 +601,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>>>  			 * the driver is unable to keep up the supply of new
>>>  			 * work).
>>>  			 */
>>> -			if (port_count(&port[1]))
>>> +			if (port_count(execlists_port_next(execlists, port)))
>>>  				goto unlock;
>>>  
>>>  			/* WaIdleLiteRestore:bdw,skl
>>> @@ -634,7 +637,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>>>  				 * combine this request with the last, then we
>>>  				 * are done.
>>>  				 */
>>> -				if (port == last_port) {
>>> +				if (port == execlists_port_tail(execlists)) {
>>>  					__list_del_many(&p->requests,
>>>  							&rq->priotree.link);
>>
>> Nothing to fix related to this patch, but I was sure next hunk was
>> going to escape my screen :) Maybe we need to cut the indents a bit.
>>
>
> I have noticed the same. But I didn't feel like attacking this loop
> until everything is in place and working.
>
>>> @@ -890,7 +902,7 @@ static void intel_lrc_irq_handler(unsigned long data)
>>>  			}
>>>  
>>>  			/* After the final element, the hw should be idle */
>>> -			GEM_BUG_ON(port_count(port) == 0 &&
>>> +			GEM_BUG_ON(port_count(execlists_port_head(execlists)) == 0 &&
>>>  				   !(status & GEN8_CTX_STATUS_ACTIVE_IDLE));
>>
>> Why did you stop trusting port variable here?
>>
>
> We did assing it pre loop before. Now we do it inside the loop. Also
> I thought I made a favour for reader (and for the bug triager
> as GEM_BUG_ON might soon show condition in dmesg) 
> to note that it is always the first port count we assert
> for idleness.

My apologies. Now rereading this it is indeed that last port
count we need to check for hw idleness.

-Mika

Patch

diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c b/drivers/gpu/drm/i915/i915_gpu_error.c
index 653fb69e7ecb..6d0bdb03b3f0 100644
--- a/drivers/gpu/drm/i915/i915_gpu_error.c
+++ b/drivers/gpu/drm/i915/i915_gpu_error.c
@@ -1333,11 +1333,13 @@  static void engine_record_requests(struct intel_engine_cs *engine,
 static void error_record_engine_execlists(struct intel_engine_cs *engine,
 					  struct drm_i915_error_engine *ee)
 {
-	const struct intel_engine_execlists * const execlists = &engine->execlists;
+	struct intel_engine_execlists * const execlists = &engine->execlists;
 	unsigned int n;
 
 	for (n = 0; n < execlists_num_ports(execlists); n++) {
-		struct drm_i915_gem_request *rq = port_request(&execlists->port[n]);
+		struct drm_i915_gem_request *rq;
+
+		rq = port_request(execlists_port(execlists, n));
 
 		if (!rq)
 			break;
diff --git a/drivers/gpu/drm/i915/i915_guc_submission.c b/drivers/gpu/drm/i915/i915_guc_submission.c
index a2e8114b739d..5222004db039 100644
--- a/drivers/gpu/drm/i915/i915_guc_submission.c
+++ b/drivers/gpu/drm/i915/i915_guc_submission.c
@@ -496,17 +496,19 @@  static void i915_guc_submit(struct intel_engine_cs *engine)
 	struct intel_guc *guc = &dev_priv->guc;
 	struct i915_guc_client *client = guc->execbuf_client;
 	struct intel_engine_execlists * const execlists = &engine->execlists;
-	struct execlist_port *port = execlists->port;
 	const unsigned int engine_id = engine->id;
 	unsigned int n;
 
 	for (n = 0; n < execlists_num_ports(execlists); n++) {
+		struct execlist_port *port;
 		struct drm_i915_gem_request *rq;
 		unsigned int count;
 
-		rq = port_unpack(&port[n], &count);
+		port = execlists_port(execlists, n);
+		rq = port_unpack(port, &count);
+
 		if (rq && count == 0) {
-			port_set(&port[n], port_pack(rq, ++count));
+			port_set(port, port_pack(rq, ++count));
 
 			if (i915_vma_is_map_and_fenceable(rq->ring->vma))
 				POSTING_READ_FW(GUC_STATUS);
@@ -561,15 +563,20 @@  static void port_assign(struct execlist_port *port,
 static void i915_guc_dequeue(struct intel_engine_cs *engine)
 {
 	struct intel_engine_execlists * const execlists = &engine->execlists;
-	struct execlist_port *port = execlists->port;
+	struct execlist_port *port;
 	struct drm_i915_gem_request *last = NULL;
-	const struct execlist_port * const last_port =
-		&execlists->port[execlists->port_mask];
 	bool submit = false;
 	struct rb_node *rb;
 
-	if (port_isset(port))
-		port++;
+	port = execlists_port_head(execlists);
+
+	/*
+	 * We don't coalesce into last submitted port with guc.
+	 * Find first free port, this is safe as we dont dequeue without
+	 * atleast last port free.
+	 */
+	while (port_isset(port))
+		port = execlists_port_next(execlists, port);
 
 	spin_lock_irq(&engine->timeline->lock);
 	rb = execlists->first;
@@ -580,7 +587,7 @@  static void i915_guc_dequeue(struct intel_engine_cs *engine)
 
 		list_for_each_entry_safe(rq, rn, &p->requests, priotree.link) {
 			if (last && rq->ctx != last->ctx) {
-				if (port == last_port) {
+				if (port == execlists_port_tail(execlists)) {
 					__list_del_many(&p->requests,
 							&rq->priotree.link);
 					goto done;
@@ -588,7 +595,8 @@  static void i915_guc_dequeue(struct intel_engine_cs *engine)
 
 				if (submit)
 					port_assign(port, last);
-				port++;
+
+				port = execlists_port_next(execlists, port);
 			}
 
 			INIT_LIST_HEAD(&rq->priotree.link);
@@ -619,22 +627,27 @@  static void i915_guc_irq_handler(unsigned long data)
 {
 	struct intel_engine_cs * const engine = (struct intel_engine_cs *)data;
 	struct intel_engine_execlists * const execlists = &engine->execlists;
-	struct execlist_port *port = execlists->port;
-	const struct execlist_port * const last_port =
-		&execlists->port[execlists->port_mask];
-	struct drm_i915_gem_request *rq;
 
-	rq = port_request(&port[0]);
-	while (rq && i915_gem_request_completed(rq)) {
+	do {
+		struct execlist_port *port;
+		struct drm_i915_gem_request *rq;
+
+		port = execlists_port_head(execlists);
+		rq = port_request(port);
+
+		if (!rq)
+			break;
+
+		if (!i915_gem_request_completed(rq))
+			break;
+
 		trace_i915_gem_request_out(rq);
 		i915_gem_request_put(rq);
 
 		execlists_port_complete(execlists, port);
+	} while (1);
 
-		rq = port_request(&port[0]);
-	}
-
-	if (!port_isset(last_port))
+	if (!port_isset(execlists_port_tail(execlists)))
 		i915_guc_dequeue(engine);
 }
 
diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c b/drivers/gpu/drm/i915/intel_engine_cs.c
index a47a9c6bea52..8ba62d4c010e 100644
--- a/drivers/gpu/drm/i915/intel_engine_cs.c
+++ b/drivers/gpu/drm/i915/intel_engine_cs.c
@@ -1549,7 +1549,7 @@  bool intel_engine_is_idle(struct intel_engine_cs *engine)
 		return false;
 
 	/* Both ports drained, no more ELSP submission? */
-	if (port_request(&engine->execlists.port[0]))
+	if (port_request(execlists_port_head(&engine->execlists)))
 		return false;
 
 	/* ELSP is empty, but there are ready requests? */
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 7f45dd7dc3e5..2945aadc4b7e 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -437,24 +437,26 @@  static inline void elsp_write(u64 desc, u32 __iomem *elsp)
 
 static void execlists_submit_ports(struct intel_engine_cs *engine)
 {
-	struct execlist_port *port = engine->execlists.port;
+	struct intel_engine_execlists * const execlists = &engine->execlists;
 	u32 __iomem *elsp =
 		engine->i915->regs + i915_mmio_reg_offset(RING_ELSP(engine));
 	unsigned int n;
 
-	for (n = execlists_num_ports(&engine->execlists); n--; ) {
+	for (n = execlists_num_ports(execlists); n--; ) {
+		struct execlist_port *port;
 		struct drm_i915_gem_request *rq;
 		unsigned int count;
 		u64 desc;
 
-		rq = port_unpack(&port[n], &count);
+		port = execlists_port(execlists, n);
+		rq = port_unpack(port, &count);
 		if (rq) {
 			GEM_BUG_ON(count > !n);
 			if (!count++)
 				execlists_context_status_change(rq, INTEL_CONTEXT_SCHEDULE_IN);
-			port_set(&port[n], port_pack(rq, count));
+			port_set(port, port_pack(rq, count));
 			desc = execlists_update_context(rq);
-			GEM_DEBUG_EXEC(port[n].context_id = upper_32_bits(desc));
+			GEM_DEBUG_EXEC(port->context_id = upper_32_bits(desc));
 		} else {
 			GEM_BUG_ON(!n);
 			desc = 0;
@@ -523,10 +525,8 @@  static bool can_preempt(struct intel_engine_cs *engine)
 static void execlists_dequeue(struct intel_engine_cs *engine)
 {
 	struct intel_engine_execlists * const execlists = &engine->execlists;
-	struct execlist_port *port = execlists->port;
-	const struct execlist_port * const last_port =
-		&execlists->port[execlists->port_mask];
-	struct drm_i915_gem_request *last = port_request(port);
+	struct execlist_port *port;
+	struct drm_i915_gem_request *last;
 	struct rb_node *rb;
 	bool submit = false;
 
@@ -557,6 +557,9 @@  static void execlists_dequeue(struct intel_engine_cs *engine)
 	if (!rb)
 		goto unlock;
 
+	port = execlists_port_head(execlists);
+	last = port_request(port);
+
 	if (last) {
 		/*
 		 * Don't resubmit or switch until all outstanding
@@ -564,7 +567,7 @@  static void execlists_dequeue(struct intel_engine_cs *engine)
 		 * know the next preemption status we see corresponds
 		 * to this ELSP update.
 		 */
-		if (port_count(&port[0]) > 1)
+		if (port_count(port) > 1)
 			goto unlock;
 
 		if (can_preempt(engine) &&
@@ -598,7 +601,7 @@  static void execlists_dequeue(struct intel_engine_cs *engine)
 			 * the driver is unable to keep up the supply of new
 			 * work).
 			 */
-			if (port_count(&port[1]))
+			if (port_count(execlists_port_next(execlists, port)))
 				goto unlock;
 
 			/* WaIdleLiteRestore:bdw,skl
@@ -634,7 +637,7 @@  static void execlists_dequeue(struct intel_engine_cs *engine)
 				 * combine this request with the last, then we
 				 * are done.
 				 */
-				if (port == last_port) {
+				if (port == execlists_port_tail(execlists)) {
 					__list_del_many(&p->requests,
 							&rq->priotree.link);
 					goto done;
@@ -658,7 +661,8 @@  static void execlists_dequeue(struct intel_engine_cs *engine)
 
 				if (submit)
 					port_assign(port, last);
-				port++;
+
+				port = execlists_port_next(execlists, port);
 
 				GEM_BUG_ON(port_isset(port));
 			}
@@ -688,19 +692,24 @@  static void execlists_dequeue(struct intel_engine_cs *engine)
 }
 
 static void
-execlist_cancel_port_requests(struct intel_engine_execlists *execlists)
+execlists_cancel_port_requests(struct intel_engine_execlists *execlists)
 {
-	struct execlist_port *port = execlists->port;
 	unsigned int num_ports = execlists_num_ports(execlists);
 
-	while (num_ports-- && port_isset(port)) {
-		struct drm_i915_gem_request *rq = port_request(port);
+	while (num_ports--) {
+		struct execlist_port *port;
+		struct drm_i915_gem_request *rq;
+
+		port = execlists_port_head(execlists);
+		if (!port_isset(port))
+			break;
+
+		rq = port_request(port);
 
 		execlists_context_status_change(rq, INTEL_CONTEXT_SCHEDULE_PREEMPTED);
 		i915_gem_request_put(rq);
 
-		memset(port, 0, sizeof(*port));
-		port++;
+		execlists_port_complete(execlists, port);
 	}
 }
 
@@ -714,7 +723,7 @@  static void execlists_cancel_requests(struct intel_engine_cs *engine)
 	spin_lock_irqsave(&engine->timeline->lock, flags);
 
 	/* Cancel the requests on the HW and clear the ELSP tracker. */
-	execlist_cancel_port_requests(execlists);
+	execlists_cancel_port_requests(execlists);
 
 	/* Mark all executing requests as skipped. */
 	list_for_each_entry(rq, &engine->timeline->requests, link) {
@@ -769,7 +778,6 @@  static void intel_lrc_irq_handler(unsigned long data)
 {
 	struct intel_engine_cs * const engine = (struct intel_engine_cs *)data;
 	struct intel_engine_execlists * const execlists = &engine->execlists;
-	struct execlist_port * const port = execlists->port;
 	struct drm_i915_private *dev_priv = engine->i915;
 
 	/* We can skip acquiring intel_runtime_pm_get() here as it was taken
@@ -788,6 +796,8 @@  static void intel_lrc_irq_handler(unsigned long data)
 	 * new request (outside of the context-switch interrupt).
 	 */
 	while (test_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted)) {
+		struct execlist_port *port;
+
 		/* The HWSP contains a (cacheable) mirror of the CSB */
 		const u32 *buf =
 			&engine->status_page.page_addr[I915_HWS_CSB_BUF0_INDEX];
@@ -855,7 +865,7 @@  static void intel_lrc_irq_handler(unsigned long data)
 
 			if (status & GEN8_CTX_STATUS_ACTIVE_IDLE &&
 			    buf[2*head + 1] == PREEMPT_ID) {
-				execlist_cancel_port_requests(execlists);
+				execlists_cancel_port_requests(execlists);
 
 				spin_lock_irq(&engine->timeline->lock);
 				unwind_incomplete_requests(engine);
@@ -870,6 +880,8 @@  static void intel_lrc_irq_handler(unsigned long data)
 			    execlists->preempt)
 				continue;
 
+			port = execlists_port_head(execlists);
+
 			/* Check the context/desc id for this event matches */
 			GEM_DEBUG_BUG_ON(buf[2 * head + 1] != port->context_id);
 
@@ -890,7 +902,7 @@  static void intel_lrc_irq_handler(unsigned long data)
 			}
 
 			/* After the final element, the hw should be idle */
-			GEM_BUG_ON(port_count(port) == 0 &&
+			GEM_BUG_ON(port_count(execlists_port_head(execlists)) == 0 &&
 				   !(status & GEN8_CTX_STATUS_ACTIVE_IDLE));
 		}
 
@@ -921,6 +933,7 @@  static void insert_request(struct intel_engine_cs *engine,
 static void execlists_submit_request(struct drm_i915_gem_request *request)
 {
 	struct intel_engine_cs *engine = request->engine;
+	struct intel_engine_execlists * const execlists = &engine->execlists;
 	unsigned long flags;
 
 	/* Will be called from irq-context when using foreign fences. */
@@ -928,7 +941,7 @@  static void execlists_submit_request(struct drm_i915_gem_request *request)
 
 	insert_request(engine, &request->priotree, request->priotree.priority);
 
-	GEM_BUG_ON(!engine->execlists.first);
+	GEM_BUG_ON(!execlists->first);
 	GEM_BUG_ON(list_empty(&request->priotree.link));
 
 	spin_unlock_irqrestore(&engine->timeline->lock, flags);
@@ -1520,7 +1533,7 @@  static void reset_common_ring(struct intel_engine_cs *engine,
 	 * guessing the missed context-switch events by looking at what
 	 * requests were completed.
 	 */
-	execlist_cancel_port_requests(execlists);
+	execlists_cancel_port_requests(execlists);
 
 	/* Push back any incomplete requests for replay after the reset. */
 	unwind_incomplete_requests(engine);
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 17186f067408..cfec73400d0f 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -251,6 +251,11 @@  struct intel_engine_execlists {
 	unsigned int port_mask;
 
 	/**
+	 * @port_head: first used execlist port
+	 */
+	unsigned int port_head;
+
+	/**
 	 * @queue: queue of requests, in priority lists
 	 */
 	struct rb_root queue;
@@ -531,6 +536,39 @@  execlists_num_ports(const struct intel_engine_execlists * const execlists)
 	return execlists->port_mask + 1;
 }
 
+#define __port_add(start, n, mask) (((start) + (n)) & (mask))
+#define port_head_add(e, n) __port_add((e)->port_head, n, (e)->port_mask)
+
+/* Index starting from port_head */
+static inline struct execlist_port *
+execlists_port(struct intel_engine_execlists * const execlists,
+	       const unsigned int n)
+{
+	return &execlists->port[port_head_add(execlists, n)];
+}
+
+static inline struct execlist_port *
+execlists_port_head(struct intel_engine_execlists * const execlists)
+{
+	return execlists_port(execlists, 0);
+}
+
+static inline struct execlist_port *
+execlists_port_tail(struct intel_engine_execlists * const execlists)
+{
+	return execlists_port(execlists, -1);
+}
+
+static inline struct execlist_port *
+execlists_port_next(struct intel_engine_execlists * const execlists,
+		    const struct execlist_port * const port)
+{
+	const unsigned int n = __port_add(port_index(port, execlists),
+					  1,
+					  execlists->port_mask);
+	return &execlists->port[n];
+}
+
 static inline void
 execlists_port_complete(struct intel_engine_execlists * const execlists,
 			struct execlist_port * const port)