Re: [Intel-gfx] [PATCH 07/11] drm/i915: Add i915_gem_context_is_full_ppgtt

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Sep 02, 2021 at 05:05:05PM +0000, Bloomfield, Jon wrote:
> > -----Original Message-----
> > From: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> > Sent: Thursday, September 2, 2021 9:42 AM
> > To: Daniel Vetter <daniel@ffwll.ch>
> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>; DRI Development <dri-
> > devel@lists.freedesktop.org>; Intel Graphics Development <intel-
> > gfx@lists.freedesktop.org>; Maarten Lankhorst
> > <maarten.lankhorst@linux.intel.com>; Vetter, Daniel
> > <daniel.vetter@intel.com>; Bloomfield, Jon <jon.bloomfield@intel.com>;
> > Chris Wilson <chris@chris-wilson.co.uk>; Joonas Lahtinen
> > <joonas.lahtinen@linux.intel.com>; Thomas Hellström
> > <thomas.hellstrom@linux.intel.com>; Auld, Matthew
> > <matthew.auld@intel.com>; Landwerlin, Lionel G
> > <lionel.g.landwerlin@intel.com>; Dave Airlie <airlied@redhat.com>; Jason
> > Ekstrand <jason@jlekstrand.net>
> > Subject: Re: [Intel-gfx] [PATCH 07/11] drm/i915: Add
> > i915_gem_context_is_full_ppgtt
> > 
> > 
> > On 02/09/2021 16:22, Daniel Vetter wrote:
> > > On Thu, Sep 02, 2021 at 03:54:36PM +0100, Tvrtko Ursulin wrote:
> > >> On 02/09/2021 15:20, Daniel Vetter wrote:
> > >>> And use it anywhere we have open-coded checks for ctx->vm that really
> > >>> only check for full ppgtt.
> > >>>
> > >>> Plus for paranoia add a GEM_BUG_ON that checks it's really only set
> > >>> when we have full ppgtt, just in case. gem_context->vm is different
> > >>> since it's NULL in ggtt mode, unlike intel_context->vm or gt->vm,
> > >>> which is always set.
> > >>>
> > >>> v2: 0day found a testcase that I missed.
> > >>>
> > >>> Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > >>> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > >>> Cc: Jon Bloomfield <jon.bloomfield@intel.com>
> > >>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > >>> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > >>> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > >>> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > >>> Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
> > >>> Cc: Matthew Auld <matthew.auld@intel.com>
> > >>> Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> > >>> Cc: Dave Airlie <airlied@redhat.com>
> > >>> Cc: Jason Ekstrand <jason@jlekstrand.net>
> > >>> ---
> > >>>    drivers/gpu/drm/i915/gem/i915_gem_context.c           | 2 +-
> > >>>    drivers/gpu/drm/i915/gem/i915_gem_context.h           | 7 +++++++
> > >>>    drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c        | 2 +-
> > >>>    drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c | 6 +++---
> > >>>    4 files changed, 12 insertions(+), 5 deletions(-)
> > >>>
> > >>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > >>> index 7a566fb7cca4..1eec85944c1f 100644
> > >>> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > >>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > >>> @@ -1566,7 +1566,7 @@ static int get_ppgtt(struct
> > drm_i915_file_private *file_priv,
> > >>>    	int err;
> > >>>    	u32 id;
> > >>> -	if (!rcu_access_pointer(ctx->vm))
> > >>> +	if (!i915_gem_context_is_full_ppgtt(ctx))
> > >>
> > >> It reads a bit wrong because GEM context cannot *be* full ppggt. It can
> > be
> > >> associated with a VM which is or isn't full ppgtt. So a test on a VM
> > >> retrieved from a context is semnntically more correct. Perhaps you want
> > to
> > >> consider adding a helper to that effect instead? It could mean splitting
> > >> into two helpers (getter + test) or maybe just renaming would work. Like
> > >> i915_gem_context_has_full_ppgtt_vm(ctx)?
> > >
> > > The pointer isn't set when the driver/context isn't running in full ppgtt
> > > mode. This is why I've added the GEM_BUG_ON to check we're not
> > breaking
> > > any invariants. So yeah it is a full ppgtt context or it's not, that is
> > > indeed the question here.
> > >
> > > I'm happy to bikeshed the naming, but I don't see how your suggestion is
> > > an improvement.
> > 
> > I think the pointer being set or not is implementation detail, for
> > instance we could have it always set just like it is in intel_context.
> > 
> > I simply think GEM context *isn't* full ppgtt, but the VM is. And since
> > GEM context *points* to a VM, *has* is the right verb in my mind. You
> > did not write why do you not see has as more correct than is so I don't
> > want to be guessing too much.
> 
> FWIW, I agree with Tvrtko. i915_gem_context_is_full_ppgtt is incorrect
> grammar. It IS a bike shed, but, hey it'll live for a while.

Generally all our feature checks are of the various is_foo variety.
HAS_FULL_PPGTT is one of the very rare exceptions. So it's a question of
"is gem_ctx foo", not "has gem_ctx foo". The fact that we implement the
check by looking at a pointer doesn't matter. And yes if it is full ppgtt,
it also has it's own private vm, but we don't care about that part at all.
It's just a distraction.

Anyway I'll repaint, least because the HAS_FULL_PPGTT thing is almost a
decade old by now :-)
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch