[PATCH 4/4] drm/i915/uapi: reject set_domain for discrete

[Jason Ekstrand]

On Fri, Jul 16, 2021 at 9:52 AM Tvrtko Ursulin
<tvrtko.ursulin at linux.intel.com> wrote:
>
>
> On 15/07/2021 11:15, Matthew Auld wrote:
> > The CPU domain should be static for discrete, and on DG1 we don't need
> > any flushing since everything is already coherent, so really all this
> > does is an object wait, for which we have an ioctl. Longer term the
> > desired caching should be an immutable creation time property for the
> > BO, which can be set with something like gem_create_ext.
> >
> > One other user is iris + userptr, which uses the set_domain to probe all
> > the pages to check if the GUP succeeds, however we now have a PROBE
> > flag for this purpose.
> >
> > v2: add some more kernel doc, also add the implicit rules with caching
> >
> > Suggested-by: Daniel Vetter <daniel at ffwll.ch>
> > Signed-off-by: Matthew Auld <matthew.auld at intel.com>
> > Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> > Cc: Jordan Justen <jordan.l.justen at intel.com>
> > Cc: Kenneth Graunke <kenneth at whitecape.org>
> > Cc: Jason Ekstrand <jason at jlekstrand.net>
> > Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> > Cc: Ramalingam C <ramalingam.c at intel.com>
> > Reviewed-by: Ramalingam C <ramalingam.c at intel.com>
> > ---
> >   drivers/gpu/drm/i915/gem/i915_gem_domain.c |  3 +++
> >   include/uapi/drm/i915_drm.h                | 19 +++++++++++++++++++
> >   2 files changed, 22 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_domain.c b/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> > index 43004bef55cb..b684a62bf3b0 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> > @@ -490,6 +490,9 @@ i915_gem_set_domain_ioctl(struct drm_device *dev, void *data,
> >       u32 write_domain = args->write_domain;
> >       int err;
> >
> > +     if (IS_DGFX(to_i915(dev)))
> > +             return -ENODEV;
> > +
> >       /* Only handle setting domains to types used by the CPU. */
> >       if ((write_domain | read_domains) & I915_GEM_GPU_DOMAINS)
> >               return -EINVAL;
> > diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> > index 2e4112bf4d38..04ce310e7ee6 100644
> > --- a/include/uapi/drm/i915_drm.h
> > +++ b/include/uapi/drm/i915_drm.h
> > @@ -901,6 +901,25 @@ struct drm_i915_gem_mmap_offset {
> >    *  - I915_GEM_DOMAIN_GTT: Mappable aperture domain
> >    *
> >    * All other domains are rejected.
> > + *
> > + * Note that for discrete, starting from DG1, this is no longer supported, and
> > + * is instead rejected. On such platforms the CPU domain is effectively static,
> > + * where we also only support a single &drm_i915_gem_mmap_offset cache mode,
> > + * which can't be set explicitly and instead depends on the object placements,
> > + * as per the below.
> > + *
> > + * Implicit caching rules, starting from DG1:
> > + *
> > + *   - If any of the object placements (see &drm_i915_gem_create_ext_memory_regions)
> > + *     contain I915_MEMORY_CLASS_DEVICE then the object will be allocated and
> > + *     mapped as write-combined only.

Is this accurate?  I thought they got WB when living in SMEM and WC
when on the device.  But, since both are coherent, it's safe to lie to
userspace and say it's all WC.  Is that correct or am I missing
something?

> A note about write-combine buffer? I guess saying it is userspace
> responsibility to do it and how.

What exactly are you thinking is userspace's responsibility?

> > + *
> > + *   - Everything else is always allocated and mapped as write-back, with the
> > + *     guarantee that everything is also coherent with the GPU.
>
> Haven't been following this so just a question on this one - it is not
> considered interesting to offer non-coherent modes, or even write
> combine, with system memory buffers, for a specific reason?

We only care about non-coherent modes on integrated little-core.
There, we share memory between CPU and GPU but snooping from the GPU
is optional.  Depending on access patterns, we might want WB with GPU
snooping or we might want WC.  I don't think we care about WC for SMEM
allocations on discrete.  For that matter, I'm not sure you can
actually shut snooping off when going across a "real" PCIe bus.  At
least not with DG1.

--Jason

> Regards,
>
> Tvrtko
>
> > + *
> > + * Note that this is likely to change in the future again, where we might need
> > + * more flexibility on future devices, so making this all explicit as part of a
> > + * new &drm_i915_gem_create_ext extension is probable.
> >    */
> >   struct drm_i915_gem_set_domain {
> >       /** @handle: Handle for the object. */
> >