Re: [Intel-gfx] [PATCH] drm: Explicitly compute the last cacheline for clflush on range

From: Daniel Vetter <daniel@ffwll.ch>
To: Chris Wilson <chris@chris-wilson.co.uk>,
	Imre Deak <imre.deak@intel.com>,
	intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
	Daniel Vetter <daniel.vetter@ffwll.ch>
Subject: Re: [Intel-gfx] [PATCH] drm: Explicitly compute the last cacheline for clflush on range
Date: Mon, 19 Oct 2015 10:35:45 +0200	[thread overview]
Message-ID: <20151019083545.GN13786@phenom.ffwll.local> (raw)
In-Reply-To: <20151018160706.GE27143@nuc-i3427.alporthouse.com>

On Sun, Oct 18, 2015 at 05:07:06PM +0100, Chris Wilson wrote:
> On Sun, Oct 18, 2015 at 02:07:13PM +0100, Chris Wilson wrote:
> > > I couldn't spot the difference either. I am beginning to suspect it is
> > > gcc as
> > > 
> > > diff --git a/drivers/gpu/drm/drm_cache.c b/drivers/gpu/drm/drm_cache.c
> > > index 6743ff7..c9097b5 100644
> > > --- a/drivers/gpu/drm/drm_cache.c
> > > +++ b/drivers/gpu/drm/drm_cache.c
> > > @@ -130,11 +130,11 @@ 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;
> > > +               void *end = addr + length - 1;
> > >                 addr = (void *)(((unsigned long)addr) & -size);
> > >                 mb();
> > > -               for (; addr < end; addr += size)
> > > +               for (; addr <= end; addr += size)
> > >                         clflushopt(addr);
> > >                 mb();
> > >                 return;
> > 
> > s/clflushopt/clflush/ works just as well.
> > 
> > Plot thickens. Current guess is that gcc doesn't see the constraints
> > underneath the alternative()?
> 
> Adding a barrier() after clflushopt() in the loop is sufficient as well.
> Almost certain that alternative() is confusing gcc.

So adding that barrier() to clflushopt with a massive comment that gcc
gets confused?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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