Re: [PATCH 5/7] [media] mipi-csis: make sparse happy

[Arnd Bergmann <arnd@arndb.de>]

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.

	Arnd