On 05/10/15 13:07, Arnd Bergmann wrote:
> On Monday 05 October 2015 12:24:40 Sylwester Nawrocki wrote:
>> > On 03/10/15 00:25, Arnd Bergmann wrote:
>>> > > On Thursday 01 October 2015 19:17:27 Mauro Carvalho Chehab wrote:
>>>>> > >> > diff --git a/drivers/media/platform/exynos4-is/mipi-csis.c b/drivers/media/platform/exynos4-is/mipi-csis.c
>>>>> > >> > index d74e1bec3d86..4b85105dc159 100644
>>>>> > >> > --- a/drivers/media/platform/exynos4-is/mipi-csis.c
>>>>> > >> > +++ b/drivers/media/platform/exynos4-is/mipi-csis.c
>>>>> > >> > @@ -706,7 +706,8 @@ static irqreturn_t s5pcsis_irq_handler(int irq, void *dev_id)
>>>>> > >> > else
>>>>> > >> > offset = S5PCSIS_PKTDATA_ODD;
>>>>> > >> >
>>>>> > >> > - memcpy(pktbuf->data, state->regs + offset, pktbuf->len);
>>>>> > >> > + memcpy(pktbuf->data, (u8 __force *)state->regs + offset,
>>>>> > >> > + pktbuf->len);
>>>>> > >> > pktbuf->data = NULL;
>>>>> > >> >
>>> > >
>>> > > I think this is what memcpy_toio() is meant for.
>> >
>> > Exactly memcpy_fromio(). But it's implementation is inefficient on
>> > ARCH=arm, memcpy_fromio() will be translated to a loop of readb(),
>> > only if an arm sub-architecture provides a processor instruction
>> > to access memory by byte. Each readb() also involves a memory barrier.
>> > That's all what we wanted to avoid. AFAIR using memcpy_fromio() was
>> > causing increase of the copy operation several times comparing to
>> > memcpy(). On arm64 it looks better, but this driver is currently
>> > used only on arm32.
>> >
>> > I would prefer to add (void __force *) instead:
>> >
>> > memcpy(pktbuf->data, (void __force *)state->regs + offset, pktbuf->len);
>> >
>> > Alternatively, the memset could just be replaced by a loop of
>> > u32 reads - __raw_readl();
>
> You are right for old kernels, but this was fixed in 7ddfe625cb ("ARM:
> optimize memset_io()/memcpy_fromio()/memcpy_toio()") at least for
> little-endian kernels and should be fine now on ARM just like
> everywhere else.
Indeed, I had just previously checked it in 4.0 kernel and missed those
recent further optimizations. It should be fine to replace memcpy()
with memcpy_fromio() then.
--
Thanks,
Sylwester
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