| Message ID | 1523457879-9869-2-git-send-email-pierre-yves.mordret@st.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 11/04/18 15:44, Pierre-Yves MORDRET wrote: > Both buffer Transfer Length (TLEN if any) and transfer size have to be > aligned on burst size (burst beats*bus width). > > Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com> > --- > Version history: > v1: > * Initial > v2: > --- > --- > drivers/dma/stm32-mdma.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/dma/stm32-mdma.c b/drivers/dma/stm32-mdma.c > index daa1602..fbcffa2 100644 > --- a/drivers/dma/stm32-mdma.c > +++ b/drivers/dma/stm32-mdma.c > @@ -413,7 +413,7 @@ static u32 stm32_mdma_get_best_burst(u32 buf_len, u32 tlen, u32 max_burst, > u32 best_burst = max_burst; > u32 burst_len = best_burst * width; > > - while ((burst_len > 0) && (tlen % burst_len)) { > + while ((burst_len > 0) && (((tlen | buf_len) & (burst_len - 1)) != 0)) { > best_burst = best_burst >> 1; > burst_len = best_burst * width; > } FWIW, doesn't that whole loop come down to just: burst_len = min(ffs(tlen | buf_len), max_burst * width); ? Robin.
Hi Robin On 04/11/2018 05:14 PM, Robin Murphy wrote: > On 11/04/18 15:44, Pierre-Yves MORDRET wrote: >> Both buffer Transfer Length (TLEN if any) and transfer size have to be >> aligned on burst size (burst beats*bus width). >> >> Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com> >> --- >> Version history: >> v1: >> * Initial >> v2: >> --- >> --- >> drivers/dma/stm32-mdma.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/drivers/dma/stm32-mdma.c b/drivers/dma/stm32-mdma.c >> index daa1602..fbcffa2 100644 >> --- a/drivers/dma/stm32-mdma.c >> +++ b/drivers/dma/stm32-mdma.c >> @@ -413,7 +413,7 @@ static u32 stm32_mdma_get_best_burst(u32 buf_len, u32 tlen, u32 max_burst, >> u32 best_burst = max_burst; >> u32 burst_len = best_burst * width; >> >> - while ((burst_len > 0) && (tlen % burst_len)) { >> + while ((burst_len > 0) && (((tlen | buf_len) & (burst_len - 1)) != 0)) { >> best_burst = best_burst >> 1; >> burst_len = best_burst * width; >> } > > FWIW, doesn't that whole loop come down to just: > > burst_len = min(ffs(tlen | buf_len), max_burst * width); No sure it ends as expected. or I miss something or don't understand this statement I tried with "relevant value" : i.e. best_burst = 32, Tlen=128(default) and buf_len = 64, width= 4. This statements gets me something wrong output => 7 instead of 16 * 4. I doubt :) > > ? > > Robin. >
On 13/04/18 10:45, Pierre Yves MORDRET wrote: > Hi Robin > > On 04/11/2018 05:14 PM, Robin Murphy wrote: >> On 11/04/18 15:44, Pierre-Yves MORDRET wrote: >>> Both buffer Transfer Length (TLEN if any) and transfer size have to be >>> aligned on burst size (burst beats*bus width). >>> >>> Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com> >>> --- >>> Version history: >>> v1: >>> * Initial >>> v2: >>> --- >>> --- >>> drivers/dma/stm32-mdma.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/drivers/dma/stm32-mdma.c b/drivers/dma/stm32-mdma.c >>> index daa1602..fbcffa2 100644 >>> --- a/drivers/dma/stm32-mdma.c >>> +++ b/drivers/dma/stm32-mdma.c >>> @@ -413,7 +413,7 @@ static u32 stm32_mdma_get_best_burst(u32 buf_len, u32 tlen, u32 max_burst, >>> u32 best_burst = max_burst; >>> u32 burst_len = best_burst * width; >>> >>> - while ((burst_len > 0) && (tlen % burst_len)) { >>> + while ((burst_len > 0) && (((tlen | buf_len) & (burst_len - 1)) != 0)) { >>> best_burst = best_burst >> 1; >>> burst_len = best_burst * width; >>> } >> >> FWIW, doesn't that whole loop come down to just: >> >> burst_len = min(ffs(tlen | buf_len), max_burst * width); > > No sure it ends as expected. or I miss something or don't understand this statement > I tried with "relevant value" : i.e. best_burst = 32, Tlen=128(default) and > buf_len = 64, width= 4. This statements gets me something wrong output => 7 > instead of 16 * 4. > I doubt :) Heh, seems I confused myself halfway through and started thinking max_burst and width were the exponents x rather than the values 2^x... A more representative guess should be: min(1 << __ffs(tlen | buf_len), max_burst * width); but the general point I was trying to make is that a loop checking whether the bottom n bits of something are zero for different values of n is unnecessary when n can simply be calculated directly*. Robin. * in the case of this "just the lowest set bit" idiom there's also the shift-free ((x & (x - 1)) ^ x), but as well as being unreadable it's generally less efficient than (1 << __ffs(x)) for most modern ISAs.
On 04/13/2018 01:09 PM, Robin Murphy wrote: > On 13/04/18 10:45, Pierre Yves MORDRET wrote: >> Hi Robin >> >> On 04/11/2018 05:14 PM, Robin Murphy wrote: >>> On 11/04/18 15:44, Pierre-Yves MORDRET wrote: >>>> Both buffer Transfer Length (TLEN if any) and transfer size have to be >>>> aligned on burst size (burst beats*bus width). >>>> >>>> Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com> >>>> --- >>>> Version history: >>>> v1: >>>> * Initial >>>> v2: >>>> --- >>>> --- >>>> drivers/dma/stm32-mdma.c | 2 +- >>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>> >>>> diff --git a/drivers/dma/stm32-mdma.c b/drivers/dma/stm32-mdma.c >>>> index daa1602..fbcffa2 100644 >>>> --- a/drivers/dma/stm32-mdma.c >>>> +++ b/drivers/dma/stm32-mdma.c >>>> @@ -413,7 +413,7 @@ static u32 stm32_mdma_get_best_burst(u32 buf_len, u32 tlen, u32 max_burst, >>>> u32 best_burst = max_burst; >>>> u32 burst_len = best_burst * width; >>>> >>>> - while ((burst_len > 0) && (tlen % burst_len)) { >>>> + while ((burst_len > 0) && (((tlen | buf_len) & (burst_len - 1)) != 0)) { >>>> best_burst = best_burst >> 1; >>>> burst_len = best_burst * width; >>>> } >>> >>> FWIW, doesn't that whole loop come down to just: >>> >>> burst_len = min(ffs(tlen | buf_len), max_burst * width); >> >> No sure it ends as expected. or I miss something or don't understand this statement >> I tried with "relevant value" : i.e. best_burst = 32, Tlen=128(default) and >> buf_len = 64, width= 4. This statements gets me something wrong output => 7 >> instead of 16 * 4. >> I doubt :) > > Heh, seems I confused myself halfway through and started thinking > max_burst and width were the exponents x rather than the values 2^x... > > A more representative guess should be: > > min(1 << __ffs(tlen | buf_len), max_burst * width); > > but the general point I was trying to make is that a loop checking > whether the bottom n bits of something are zero for different values of > n is unnecessary when n can simply be calculated directly*. > > Robin. Got the point. I figure how to compute this value with __ffs. Your last statement doesn't provide the good value, but the spirit is here. I just have to adjust with what I want. Thx > > > * in the case of this "just the lowest set bit" idiom there's also the > shift-free ((x & (x - 1)) ^ x), but as well as being unreadable it's > generally less efficient than (1 << __ffs(x)) for most modern ISAs. >
diff --git a/drivers/dma/stm32-mdma.c b/drivers/dma/stm32-mdma.c index daa1602..fbcffa2 100644 --- a/drivers/dma/stm32-mdma.c +++ b/drivers/dma/stm32-mdma.c @@ -413,7 +413,7 @@ static u32 stm32_mdma_get_best_burst(u32 buf_len, u32 tlen, u32 max_burst, u32 best_burst = max_burst; u32 burst_len = best_burst * width; - while ((burst_len > 0) && (tlen % burst_len)) { + while ((burst_len > 0) && (((tlen | buf_len) & (burst_len - 1)) != 0)) { best_burst = best_burst >> 1; burst_len = best_burst * width; }
Both buffer Transfer Length (TLEN if any) and transfer size have to be aligned on burst size (burst beats*bus width). Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com> --- Version history: v1: * Initial v2: --- --- drivers/dma/stm32-mdma.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)