Re: [PATCH] drm: Avoid the double clflush on the last cache line in drm_clflush_virt_range()

[Chris Wilson <chris@chris-wilson.co.uk>]

On Thu, Jun 11, 2015 at 09:25:16AM +0100, Dave Gordon wrote:
> On 10/06/15 15:58, Chris Wilson wrote:
> > As the clflush operates on cache lines, and we can flush any byte
> > address, in order to flush all bytes given in the range we issue an
> > extra clflush on the last byte to ensure the last cacheline is flushed.
> > We can can the iteration to be over the actual cache lines to avoid this
> > double clflush on the last byte.
> > 
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Imre Deak <imre.deak@intel.com>
> > ---
> >  drivers/gpu/drm/drm_cache.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_cache.c b/drivers/gpu/drm/drm_cache.c
> > index 9a62d7a53553..6743ff7dccfa 100644
> > --- a/drivers/gpu/drm/drm_cache.c
> > +++ b/drivers/gpu/drm/drm_cache.c
> > @@ -130,11 +130,12 @@ drm_clflush_virt_range(void *addr, unsigned long length)
> >  {
> >  #if defined(CONFIG_X86)
> >  	if (cpu_has_clflush) {
> > +		const int size = boot_cpu_data.x86_clflush_size;
> >  		void *end = addr + length;
> > +		addr = (void *)(((unsigned long)addr) & -size);
> 
> Should this cast be to uintptr_t?

The kernel has a strict equivalence between sizeof(unsigned long) and
sizeof(pointer). You will see unsigned long used universally to pass
along pointers to functions and as closures.

> Or intptr_t, as size has somewhat
> strangely been defined as signed? To complete the mix, x86_clflush_size
> is 'u16'! So maybe we should write
> 
> +		const size_t size = boot_cpu_data.x86_clflush_size;
> +		const size_t mask = ~(size - 1);
>  		void *end = addr + length;
> +		addr = (void *)(((uintptr_t)addr) & mask);

No. size_t has very poor definition inside the kernel - what does the
maximum size of a userspace allocation have to do with kernel internals?

Let's keep userspace types in userspace, or else we end up with
i915_gem_gtt.c.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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