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

[Imre Deak <imre.deak@intel.com>]

On to, 2015-06-11 at 09:33 +0100, Chris Wilson wrote:
> 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.

I also think using unsigned long for virtual addresses is standard in
the kernel and I can't see how using int would lead to problems given
the expected range of x86_clflush_size, so this looks ok to me:
Reviewed-by: Imre Deak <imre.deak@intel.com>

> -Chris
> 


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