[v3] clk: ti: omap36xx: Work around sprz319 advisory 2.1

Message ID	1480713278-6884-1-git-send-email-laurent.pinchart@ideasonboard.com (mailing list archive)
State	Accepted, archived
Delegated to:	Stephen Boyd
Headers	show Return-Path: <linux-clk-owner@kernel.org> From: Laurent Pinchart <laurent.pinchart@ideasonboard.com> To: linux-omap@vger.kernel.org Cc: linux-clk@vger.kernel.org, Paul Walmsley <paul@pwsan.com>, Tero Kristo <t-kristo@ti.com>, Richard Watts <rrw@kynesim.co.uk>, Tony Lindgren <tony@atomide.com>, Alexander Kinzer <a.kinzer@plusoptix.de> Subject: [PATCH v3] clk: ti: omap36xx: Work around sprz319 advisory 2.1 Date: Fri, 2 Dec 2016 23:14:38 +0200 Message-Id: <1480713278-6884-1-git-send-email-laurent.pinchart@ideasonboard.com> Sender: linux-clk-owner@vger.kernel.org Precedence: bulk

Laurent Pinchart Dec. 2, 2016, 9:14 p.m. UTC

From: Richard Watts <rrw@kynesim.co.uk>

The OMAP36xx DPLL5, driving EHCI USB, can be subject to a long-term
frequency drift. The frequency drift magnitude depends on the VCO update
rate, which is inversely proportional to the PLL divider. The kernel
DPLL configuration code results in a high value for the divider, leading
to a long term drift high enough to cause USB transmission errors. In
the worst case the USB PHY's ULPI interface can stop responding,
breaking USB operation completely. This manifests itself on the
Beagleboard xM by the LAN9514 reporting 'Cannot enable port 2. Maybe the
cable is bad?' in the kernel log.

Errata sprz319 advisory 2.1 documents PLL values that minimize the
drift. Use them automatically when DPLL5 is used for USB operation,
which we detect based on the requested clock rate. The clock framework
will still compute the PLL parameters and resulting rate as usual, but
the PLL M and N values will then be overridden. This can result in the
effective clock rate being slightly different than the rate cached by
the clock framework, but won't cause any adverse effect to USB
operation.

Signed-off-by: Richard Watts <rrw@kynesim.co.uk>
[Upported from v3.2 to v4.9]
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
---
Changes since v2:

- Added spaces around +
---
 drivers/clk/ti/clk-3xxx.c | 20 +++++++-------
 drivers/clk/ti/clock.h    |  9 +++++++
 drivers/clk/ti/dpll.c     | 19 +++++++++++++-
 drivers/clk/ti/dpll3xxx.c | 67 +++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 104 insertions(+), 11 deletions(-)

Michael Turquette Dec. 2, 2016, 10:41 p.m. UTC | #1

Quoting Laurent Pinchart (2016-12-02 13:14:38)
> From: Richard Watts <rrw@kynesim.co.uk>
> 
> The OMAP36xx DPLL5, driving EHCI USB, can be subject to a long-term
> frequency drift. The frequency drift magnitude depends on the VCO update
> rate, which is inversely proportional to the PLL divider. The kernel
> DPLL configuration code results in a high value for the divider, leading
> to a long term drift high enough to cause USB transmission errors. In
> the worst case the USB PHY's ULPI interface can stop responding,
> breaking USB operation completely. This manifests itself on the
> Beagleboard xM by the LAN9514 reporting 'Cannot enable port 2. Maybe the
> cable is bad?' in the kernel log.
> 
> Errata sprz319 advisory 2.1 documents PLL values that minimize the
> drift. Use them automatically when DPLL5 is used for USB operation,
> which we detect based on the requested clock rate. The clock framework
> will still compute the PLL parameters and resulting rate as usual, but
> the PLL M and N values will then be overridden. This can result in the
> effective clock rate being slightly different than the rate cached by
> the clock framework, but won't cause any adverse effect to USB
> operation.
> 
> Signed-off-by: Richard Watts <rrw@kynesim.co.uk>
> [Upported from v3.2 to v4.9]
> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> ---
> Changes since v2:
> 
> - Added spaces around +
> ---
>  drivers/clk/ti/clk-3xxx.c | 20 +++++++-------
>  drivers/clk/ti/clock.h    |  9 +++++++
>  drivers/clk/ti/dpll.c     | 19 +++++++++++++-
>  drivers/clk/ti/dpll3xxx.c | 67 +++++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 104 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/clk/ti/clk-3xxx.c b/drivers/clk/ti/clk-3xxx.c
> index 8831e1a05367..11d8aa3ec186 100644
> --- a/drivers/clk/ti/clk-3xxx.c
> +++ b/drivers/clk/ti/clk-3xxx.c
> @@ -22,13 +22,6 @@
>  
>  #include "clock.h"
>  
> -/*
> - * DPLL5_FREQ_FOR_USBHOST: USBHOST and USBTLL are the only clocks
> - * that are sourced by DPLL5, and both of these require this clock
> - * to be at 120 MHz for proper operation.
> - */
> -#define DPLL5_FREQ_FOR_USBHOST         120000000
> -
>  #define OMAP3430ES2_ST_DSS_IDLE_SHIFT                  1
>  #define OMAP3430ES2_ST_HSOTGUSB_IDLE_SHIFT             5
>  #define OMAP3430ES2_ST_SSI_IDLE_SHIFT                  8
> @@ -546,14 +539,21 @@ void __init omap3_clk_lock_dpll5(void)
>         struct clk *dpll5_clk;
>         struct clk *dpll5_m2_clk;
>  
> +       /*
> +        * Errata sprz319f advisory 2.1 documents a USB host clock drift issue
> +        * that can be worked around using specially crafted dpll5 settings
> +        * with a dpll5_m2 divider set to 8. Set the dpll5 rate to 8x the USB
> +        * host clock rate, its .set_rate handler() will detect that frequency
> +        * and use the errata settings.
> +        */
>         dpll5_clk = clk_get(NULL, "dpll5_ck");
> -       clk_set_rate(dpll5_clk, DPLL5_FREQ_FOR_USBHOST);
> +       clk_set_rate(dpll5_clk, OMAP3_DPLL5_FREQ_FOR_USBHOST * 8);
>         clk_prepare_enable(dpll5_clk);
>  
> -       /* Program dpll5_m2_clk divider for no division */
> +       /* Program dpll5_m2_clk divider */
>         dpll5_m2_clk = clk_get(NULL, "dpll5_m2_ck");
>         clk_prepare_enable(dpll5_m2_clk);
> -       clk_set_rate(dpll5_m2_clk, DPLL5_FREQ_FOR_USBHOST);
> +       clk_set_rate(dpll5_m2_clk, OMAP3_DPLL5_FREQ_FOR_USBHOST);
>  
>         clk_disable_unprepare(dpll5_m2_clk);
>         clk_disable_unprepare(dpll5_clk);
> diff --git a/drivers/clk/ti/clock.h b/drivers/clk/ti/clock.h
> index 90f3f472ae1c..13c37f48d9d6 100644
> --- a/drivers/clk/ti/clock.h
> +++ b/drivers/clk/ti/clock.h
> @@ -257,11 +257,20 @@ long omap2_dpll_round_rate(struct clk_hw *hw, unsigned long target_rate,
>  unsigned long omap3_clkoutx2_recalc(struct clk_hw *hw,
>                                     unsigned long parent_rate);
>  
> +/*
> + * OMAP3_DPLL5_FREQ_FOR_USBHOST: USBHOST and USBTLL are the only clocks
> + * that are sourced by DPLL5, and both of these require this clock
> + * to be at 120 MHz for proper operation.
> + */
> +#define OMAP3_DPLL5_FREQ_FOR_USBHOST   120000000
> +
>  unsigned long omap3_dpll_recalc(struct clk_hw *hw, unsigned long parent_rate);
>  int omap3_dpll4_set_rate(struct clk_hw *clk, unsigned long rate,
>                          unsigned long parent_rate);
>  int omap3_dpll4_set_rate_and_parent(struct clk_hw *hw, unsigned long rate,
>                                     unsigned long parent_rate, u8 index);
> +int omap3_dpll5_set_rate(struct clk_hw *hw, unsigned long rate,
> +                        unsigned long parent_rate);
>  void omap3_clk_lock_dpll5(void);
>  
>  unsigned long omap4_dpll_regm4xen_recalc(struct clk_hw *hw,
> diff --git a/drivers/clk/ti/dpll.c b/drivers/clk/ti/dpll.c
> index 9fc8754a6e61..4b9a419d8e14 100644
> --- a/drivers/clk/ti/dpll.c
> +++ b/drivers/clk/ti/dpll.c
> @@ -114,6 +114,18 @@ static const struct clk_ops omap3_dpll_ck_ops = {
>         .round_rate     = &omap2_dpll_round_rate,
>  };
>  
> +static const struct clk_ops omap3_dpll5_ck_ops = {
> +       .enable         = &omap3_noncore_dpll_enable,
> +       .disable        = &omap3_noncore_dpll_disable,
> +       .get_parent     = &omap2_init_dpll_parent,
> +       .recalc_rate    = &omap3_dpll_recalc,
> +       .set_rate       = &omap3_dpll5_set_rate,
> +       .set_parent     = &omap3_noncore_dpll_set_parent,
> +       .set_rate_and_parent    = &omap3_noncore_dpll_set_rate_and_parent,
> +       .determine_rate = &omap3_noncore_dpll_determine_rate,
> +       .round_rate     = &omap2_dpll_round_rate,
> +};
> +
>  static const struct clk_ops omap3_dpll_per_ck_ops = {
>         .enable         = &omap3_noncore_dpll_enable,
>         .disable        = &omap3_noncore_dpll_disable,
> @@ -474,7 +486,12 @@ static void __init of_ti_omap3_dpll_setup(struct device_node *node)
>                 .modes = (1 << DPLL_LOW_POWER_BYPASS) | (1 << DPLL_LOCKED),
>         };
>  
> -       of_ti_dpll_setup(node, &omap3_dpll_ck_ops, &dd);
> +       if ((of_machine_is_compatible("ti,omap3630") ||
> +            of_machine_is_compatible("ti,omap36xx")) &&
> +           !strcmp(node->name, "dpll5_ck"))
> +               of_ti_dpll_setup(node, &omap3_dpll5_ck_ops, &dd);
> +       else
> +               of_ti_dpll_setup(node, &omap3_dpll_ck_ops, &dd);
>  }
>  CLK_OF_DECLARE(ti_omap3_dpll_clock, "ti,omap3-dpll-clock",
>                of_ti_omap3_dpll_setup);
> diff --git a/drivers/clk/ti/dpll3xxx.c b/drivers/clk/ti/dpll3xxx.c
> index 88f2ce81ba55..4cdd28a25584 100644
> --- a/drivers/clk/ti/dpll3xxx.c
> +++ b/drivers/clk/ti/dpll3xxx.c
> @@ -838,3 +838,70 @@ int omap3_dpll4_set_rate_and_parent(struct clk_hw *hw, unsigned long rate,
>         return omap3_noncore_dpll_set_rate_and_parent(hw, rate, parent_rate,
>                                                       index);
>  }
> +
> +/* Apply DM3730 errata sprz319 advisory 2.1. */
> +static bool omap3_dpll5_apply_errata(struct clk_hw *hw,
> +                                    unsigned long parent_rate)
> +{
> +       struct omap3_dpll5_settings {
> +               unsigned int rate, m, n;
> +       };
> +
> +       static const struct omap3_dpll5_settings precomputed[] = {
> +               /*
> +                * From DM3730 errata advisory 2.1, table 35 and 36.
> +                * The N value is increased by 1 compared to the tables as the
> +                * errata lists register values while last_rounded_field is the
> +                * real divider value.
> +                */
> +               { 12000000,  80,  0 + 1 },
> +               { 13000000, 443,  5 + 1 },
> +               { 19200000,  50,  0 + 1 },
> +               { 26000000, 443, 11 + 1 },
> +               { 38400000,  25,  0 + 1 }
> +       };
> +
> +       const struct omap3_dpll5_settings *d;
> +       struct clk_hw_omap *clk = to_clk_hw_omap(hw);
> +       struct dpll_data *dd;
> +       unsigned int i;
> +
> +       for (i = 0; i < ARRAY_SIZE(precomputed); ++i) {
> +               if (parent_rate == precomputed[i].rate)
> +                       break;
> +       }
> +
> +       if (i == ARRAY_SIZE(precomputed))
> +               return false;
> +
> +       d = &precomputed[i];
> +
> +       /* Update the M, N and rounded rate values and program the DPLL. */
> +       dd = clk->dpll_data;
> +       dd->last_rounded_m = d->m;
> +       dd->last_rounded_n = d->n;
> +       dd->last_rounded_rate = div_u64((u64)parent_rate * d->m, d->n);
> +       omap3_noncore_dpll_program(clk, 0);
> +
> +       return true;
> +}
> +
> +/**
> + * omap3_dpll5_set_rate - set rate for omap3 dpll5
> + * @hw: clock to change
> + * @rate: target rate for clock
> + * @parent_rate: rate of the parent clock
> + *
> + * Set rate for the DPLL5 clock. Apply the sprz319 advisory 2.1 on OMAP36xx if
> + * the DPLL is used for USB host (detected through the requested rate).
> + */
> +int omap3_dpll5_set_rate(struct clk_hw *hw, unsigned long rate,
> +                        unsigned long parent_rate)
> +{
> +       if (rate == OMAP3_DPLL5_FREQ_FOR_USBHOST * 8) {
> +               if (omap3_dpll5_apply_errata(hw, parent_rate))
> +                       return 0;
> +       }
> +
> +       return omap3_noncore_dpll_set_rate(hw, rate, parent_rate);

Looks OK to me. This part is a bit sketchy, relying on the requested
rate to determine whether or not to deal with the drift, but errata are
often a sketchy affair.

Regards,
Mike

> +}
> -- 
> Regards,
> 
> Laurent Pinchart
> 
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Stephen Boyd Dec. 8, 2016, 9:16 p.m. UTC | #2

On 12/02, Laurent Pinchart wrote:
> From: Richard Watts <rrw@kynesim.co.uk>
> 
> The OMAP36xx DPLL5, driving EHCI USB, can be subject to a long-term
> frequency drift. The frequency drift magnitude depends on the VCO update
> rate, which is inversely proportional to the PLL divider. The kernel
> DPLL configuration code results in a high value for the divider, leading
> to a long term drift high enough to cause USB transmission errors. In
> the worst case the USB PHY's ULPI interface can stop responding,
> breaking USB operation completely. This manifests itself on the
> Beagleboard xM by the LAN9514 reporting 'Cannot enable port 2. Maybe the
> cable is bad?' in the kernel log.
> 
> Errata sprz319 advisory 2.1 documents PLL values that minimize the
> drift. Use them automatically when DPLL5 is used for USB operation,
> which we detect based on the requested clock rate. The clock framework
> will still compute the PLL parameters and resulting rate as usual, but
> the PLL M and N values will then be overridden. This can result in the
> effective clock rate being slightly different than the rate cached by
> the clock framework, but won't cause any adverse effect to USB
> operation.
> 
> Signed-off-by: Richard Watts <rrw@kynesim.co.uk>
> [Upported from v3.2 to v4.9]
> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> ---

Applied to clk-next

Laurent Pinchart Dec. 8, 2016, 9:24 p.m. UTC | #3

Hi Stephen,

On Thursday 08 Dec 2016 13:16:01 Stephen Boyd wrote:
> On 12/02, Laurent Pinchart wrote:
> > From: Richard Watts <rrw@kynesim.co.uk>
> > 
> > The OMAP36xx DPLL5, driving EHCI USB, can be subject to a long-term
> > frequency drift. The frequency drift magnitude depends on the VCO update
> > rate, which is inversely proportional to the PLL divider. The kernel
> > DPLL configuration code results in a high value for the divider, leading
> > to a long term drift high enough to cause USB transmission errors. In
> > the worst case the USB PHY's ULPI interface can stop responding,
> > breaking USB operation completely. This manifests itself on the
> > Beagleboard xM by the LAN9514 reporting 'Cannot enable port 2. Maybe the
> > cable is bad?' in the kernel log.
> > 
> > Errata sprz319 advisory 2.1 documents PLL values that minimize the
> > drift. Use them automatically when DPLL5 is used for USB operation,
> > which we detect based on the requested clock rate. The clock framework
> > will still compute the PLL parameters and resulting rate as usual, but
> > the PLL M and N values will then be overridden. This can result in the
> > effective clock rate being slightly different than the rate cached by
> > the clock framework, but won't cause any adverse effect to USB
> > operation.
> > 
> > Signed-off-by: Richard Watts <rrw@kynesim.co.uk>
> > [Upported from v3.2 to v4.9]
> > Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> > ---
> 
> Applied to clk-next

Thank you.

Adam Ford Jan. 3, 2017, 6 p.m. UTC | #4

On Thu, Dec 8, 2016 at 3:24 PM, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
> Hi Stephen,
>
> On Thursday 08 Dec 2016 13:16:01 Stephen Boyd wrote:
>> On 12/02, Laurent Pinchart wrote:
>> > From: Richard Watts <rrw@kynesim.co.uk>
>> >
>> > The OMAP36xx DPLL5, driving EHCI USB, can be subject to a long-term
>> > frequency drift. The frequency drift magnitude depends on the VCO update
>> > rate, which is inversely proportional to the PLL divider. The kernel
>> > DPLL configuration code results in a high value for the divider, leading
>> > to a long term drift high enough to cause USB transmission errors. In
>> > the worst case the USB PHY's ULPI interface can stop responding,
>> > breaking USB operation completely. This manifests itself on the
>> > Beagleboard xM by the LAN9514 reporting 'Cannot enable port 2. Maybe the
>> > cable is bad?' in the kernel log.
>> >
>> > Errata sprz319 advisory 2.1 documents PLL values that minimize the
>> > drift. Use them automatically when DPLL5 is used for USB operation,
>> > which we detect based on the requested clock rate. The clock framework
>> > will still compute the PLL parameters and resulting rate as usual, but
>> > the PLL M and N values will then be overridden. This can result in the
>> > effective clock rate being slightly different than the rate cached by
>> > the clock framework, but won't cause any adverse effect to USB
>> > operation.
>> >
>> > Signed-off-by: Richard Watts <rrw@kynesim.co.uk>
>> > [Upported from v3.2 to v4.9]
>> > Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
>> > ---
>>
>> Applied to clk-next
>

Since this fixes an errata issue, is there any way we can get this
patch applied to the older LTS kernels?  (4.4 seems to apply cleanly,
but I didn't try any older than that)

adam
> Thank you.
>
> --
> Regards,
>
> Laurent Pinchart
>
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Stephen Boyd Jan. 3, 2017, 6:49 p.m. UTC | #5

On 01/03, Adam Ford wrote:
> On Thu, Dec 8, 2016 at 3:24 PM, Laurent Pinchart
> <laurent.pinchart@ideasonboard.com> wrote:
> > Hi Stephen,
> >
> > On Thursday 08 Dec 2016 13:16:01 Stephen Boyd wrote:
> >> On 12/02, Laurent Pinchart wrote:
> >> > From: Richard Watts <rrw@kynesim.co.uk>
> >> >
> >> > The OMAP36xx DPLL5, driving EHCI USB, can be subject to a long-term
> >> > frequency drift. The frequency drift magnitude depends on the VCO update
> >> > rate, which is inversely proportional to the PLL divider. The kernel
> >> > DPLL configuration code results in a high value for the divider, leading
> >> > to a long term drift high enough to cause USB transmission errors. In
> >> > the worst case the USB PHY's ULPI interface can stop responding,
> >> > breaking USB operation completely. This manifests itself on the
> >> > Beagleboard xM by the LAN9514 reporting 'Cannot enable port 2. Maybe the
> >> > cable is bad?' in the kernel log.
> >> >
> >> > Errata sprz319 advisory 2.1 documents PLL values that minimize the
> >> > drift. Use them automatically when DPLL5 is used for USB operation,
> >> > which we detect based on the requested clock rate. The clock framework
> >> > will still compute the PLL parameters and resulting rate as usual, but
> >> > the PLL M and N values will then be overridden. This can result in the
> >> > effective clock rate being slightly different than the rate cached by
> >> > the clock framework, but won't cause any adverse effect to USB
> >> > operation.
> >> >
> >> > Signed-off-by: Richard Watts <rrw@kynesim.co.uk>
> >> > [Upported from v3.2 to v4.9]
> >> > Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> >> > ---
> >>
> >> Applied to clk-next
> >
> 
> Since this fixes an errata issue, is there any way we can get this
> patch applied to the older LTS kernels?  (4.4 seems to apply cleanly,
> but I didn't try any older than that)
> 

Sure. Just follow the stable kernel rules. See
Documentation/process/stable-kernel-rules.rst, specifically
option #2.

Laurent Pinchart Jan. 3, 2017, 10:16 p.m. UTC | #6

On Tuesday 03 Jan 2017 10:49:56 Stephen Boyd wrote:
> On 01/03, Adam Ford wrote:
> > On Thu, Dec 8, 2016 at 3:24 PM, Laurent Pinchart wrote:
> >> On Thursday 08 Dec 2016 13:16:01 Stephen Boyd wrote:
> >>> On 12/02, Laurent Pinchart wrote:
> >>>> From: Richard Watts <rrw@kynesim.co.uk>
> >>>> 
> >>>> The OMAP36xx DPLL5, driving EHCI USB, can be subject to a long-term
> >>>> frequency drift. The frequency drift magnitude depends on the VCO
> >>>> update rate, which is inversely proportional to the PLL divider. The
> >>>> kernel DPLL configuration code results in a high value for the divider,
> >>>> leading to a long term drift high enough to cause USB transmission
> >>>> errors. In the worst case the USB PHY's ULPI interface can stop
> >>>> responding, breaking USB operation completely. This manifests itself on
> >>>> the Beagleboard xM by the LAN9514 reporting 'Cannot enable port 2.
> >>>> Maybe the cable is bad?' in the kernel log.
> >>>> 
> >>>> Errata sprz319 advisory 2.1 documents PLL values that minimize the
> >>>> drift. Use them automatically when DPLL5 is used for USB operation,
> >>>> which we detect based on the requested clock rate. The clock
> >>>> framework will still compute the PLL parameters and resulting rate as
> >>>> usual, but the PLL M and N values will then be overridden. This can
> >>>> result in the effective clock rate being slightly different than the
> >>>> rate cached by the clock framework, but won't cause any adverse effect
> >>>> to USB operation.
> >>>> 
> >>>> Signed-off-by: Richard Watts <rrw@kynesim.co.uk>
> >>>> [Upported from v3.2 to v4.9]
> >>>> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> >>>> ---
> >>> 
> >>> Applied to clk-next
> > 
> > Since this fixes an errata issue, is there any way we can get this
> > patch applied to the older LTS kernels?  (4.4 seems to apply cleanly,
> > but I didn't try any older than that)
> 
> Sure. Just follow the stable kernel rules. See
> Documentation/process/stable-kernel-rules.rst, specifically
> option #2.

And needless to say, please test the backported patch before requesting it to 
be included in stable kernels :-)

Adam Ford Jan. 4, 2017, 12:59 p.m. UTC | #7

On Tue, Jan 3, 2017 at 4:16 PM, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
> On Tuesday 03 Jan 2017 10:49:56 Stephen Boyd wrote:
>> On 01/03, Adam Ford wrote:
>> > On Thu, Dec 8, 2016 at 3:24 PM, Laurent Pinchart wrote:
>> >> On Thursday 08 Dec 2016 13:16:01 Stephen Boyd wrote:
>> >>> On 12/02, Laurent Pinchart wrote:
>> >>>> From: Richard Watts <rrw@kynesim.co.uk>
>> >>>>
>> >>>> The OMAP36xx DPLL5, driving EHCI USB, can be subject to a long-term
>> >>>> frequency drift. The frequency drift magnitude depends on the VCO
>> >>>> update rate, which is inversely proportional to the PLL divider. The
>> >>>> kernel DPLL configuration code results in a high value for the divider,
>> >>>> leading to a long term drift high enough to cause USB transmission
>> >>>> errors. In the worst case the USB PHY's ULPI interface can stop
>> >>>> responding, breaking USB operation completely. This manifests itself on
>> >>>> the Beagleboard xM by the LAN9514 reporting 'Cannot enable port 2.
>> >>>> Maybe the cable is bad?' in the kernel log.
>> >>>>
>> >>>> Errata sprz319 advisory 2.1 documents PLL values that minimize the
>> >>>> drift. Use them automatically when DPLL5 is used for USB operation,
>> >>>> which we detect based on the requested clock rate. The clock
>> >>>> framework will still compute the PLL parameters and resulting rate as
>> >>>> usual, but the PLL M and N values will then be overridden. This can
>> >>>> result in the effective clock rate being slightly different than the
>> >>>> rate cached by the clock framework, but won't cause any adverse effect
>> >>>> to USB operation.
>> >>>>
>> >>>> Signed-off-by: Richard Watts <rrw@kynesim.co.uk>
>> >>>> [Upported from v3.2 to v4.9]
>> >>>> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
>> >>>> ---
>> >>>
>> >>> Applied to clk-next
>> >
>> > Since this fixes an errata issue, is there any way we can get this
>> > patch applied to the older LTS kernels?  (4.4 seems to apply cleanly,
>> > but I didn't try any older than that)
>>
>> Sure. Just follow the stable kernel rules. See
>> Documentation/process/stable-kernel-rules.rst, specifically
>> option #2.
>
> And needless to say, please test the backported patch before requesting it to
> be included in stable kernels :-)
>

Tested against linux-stable 4.4.y branch, linux-stable-4.8.y branch, and 4.9.0

:-)


> --
> Regards,
>
> Laurent Pinchart
>
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[v3] clk: ti: omap36xx: Work around sprz319 advisory 2.1

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