[Intel-gfx] [PATCH v2 2/2] drm/i915/bxt: Fix inadvertent CPU snooping due to incorrect MOCS config

[Chris Wilson]

On Tue, Apr 26, 2016 at 04:17:55PM +0300, Imre Deak wrote:
> On ti, 2016-04-26 at 13:57 +0100, Chris Wilson wrote:
> > On Tue, Apr 26, 2016 at 03:44:22PM +0300, Imre Deak wrote:
> > > Setting a write-back cache policy in the MOCS entry definition also
> > > implies snooping, which has a considerable overhead. This is
> > > unexpected for a few reasons:
> > 
> > If it is snooping, then I don't see why it is undesirable to have it
> > available in a mocs setting. If it is bogus and the bit is undefined,
> > then by all means remove it.
> 
> None of these entries are used alone for coherent surfaces. For that
> the application would have to use entry index#1 or #2 _and_ call the
> set caching IOCTL to set the corresponding buffer to be cached.

No, the application doesn't. There are sufficent interfaces exposed that
userspace can bypass the kernel if it so desired.

> The
> problem is that without setting the buffer to be cacheable the
> expectation is that we won't be snooping and incur the corresponding
> overhead. This is what this patch addresses.

Not true.

> The bit is also bogus, if we wanted snooping via MOCS we'd use the
> dedicated HW flag for that.

But you keep saying this bit *enables* snooping. So either it does or it
doesn't.

> If we wanted to have a snooping MOCS entry we should add that
> separately (as a forth entry), but we'd need this change as a fix for
> current users.

The current users who are getting what they request but don't know what
they were requesting?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre