| Message ID | 20151018122811.GC27143@nuc-i3427.alporthouse.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Sun, Oct 18, 2015 at 01:28:11PM +0100, Chris Wilson wrote: > On Sat, Oct 17, 2015 at 11:03:19PM +0300, Imre Deak wrote: > > On Fri, 2015-10-16 at 20:55 +0100, Chris Wilson wrote: > > > Fixes regression from > > > > > > commit afcd950cafea6e27b739fe7772cbbeed37d05b8b > > > Author: Chris Wilson <chris@chris-wilson.co.uk> > > > Date: Wed Jun 10 15:58:01 2015 +0100 > > > > > > drm: Avoid the double clflush on the last cache line in drm_clflush_virt_range() > > > > > > I'm stumped. Looking at the loop we should be iterating over every cache > > > line until we reach the start of the cacheline after the end of the > > > virtual range. Evidence says otherwise. > > > > > > More bizarely, I stored the last address to be clflushed and found it to > > > be equal to the start of the cacheline containing the last byte. Doubly > > > purplexed. > > > > > > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92501 > > > Testcase: gem_tiled_partial_pwrite_pread/reads > > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> > > > Cc: Imre Deak <imre.deak@intel.com> > > > Cc: Daniel Vetter <daniel.vetter@ffwll.ch> > > > --- > > > drivers/gpu/drm/drm_cache.c | 9 ++++++--- > > > 1 file changed, 6 insertions(+), 3 deletions(-) > > > > > > diff --git a/drivers/gpu/drm/drm_cache.c b/drivers/gpu/drm/drm_cache.c > > > index 6743ff7dccfa..7c909bc8b68a 100644 > > > --- a/drivers/gpu/drm/drm_cache.c > > > +++ b/drivers/gpu/drm/drm_cache.c > > > @@ -131,10 +131,13 @@ 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); > > > + void *end; > > > + > > > + end = (void *)(((unsigned long)addr + length - 1) & -size); > > > + addr = (void *)((unsigned long)addr & -size); > > > + > > > mb(); > > > - for (; addr < end; addr += size) > > > + for (; addr <= end; addr += size) > > > > Hm, I can't see how could this make any difference. The old way still > > looks ok to me and the new version would flush the exact same cache > > lines as the old one using the same addresses (beginning of each cache > > line). > > 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()? -Chris
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. -Chris
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
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;