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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

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.

Regards,

Tvrtko

>>
>> Regards,
>>
>> Tvrtko
>>
>>>    		return -ENODEV;
>>>    	rcu_read_lock();
>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.h b/drivers/gpu/drm/i915/gem/i915_gem_context.h
>>> index da6e8b506d96..37536a260e6e 100644
>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.h
>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.h
>>> @@ -154,6 +154,13 @@ i915_gem_context_vm(struct i915_gem_context *ctx)
>>>    	return rcu_dereference_protected(ctx->vm, lockdep_is_held(&ctx->mutex));
>>>    }
>>> +static inline bool i915_gem_context_is_full_ppgtt(struct i915_gem_context *ctx)
>>> +{
>>> +	GEM_BUG_ON(!!rcu_access_pointer(ctx->vm) != HAS_FULL_PPGTT(ctx->i915));
>>> +
>>> +	return !!rcu_access_pointer(ctx->vm);
>>> +}
>>> +
>>>    static inline struct i915_address_space *
>>>    i915_gem_context_get_eb_vm(struct i915_gem_context *ctx)
>>>    {
>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
>>> index 905b1cbd22d5..40f08948f0b2 100644
>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
>>> @@ -749,7 +749,7 @@ static int eb_select_context(struct i915_execbuffer *eb)
>>>    		return PTR_ERR(ctx);
>>>    	eb->gem_context = ctx;
>>> -	if (rcu_access_pointer(ctx->vm))
>>> +	if (i915_gem_context_is_full_ppgtt(ctx))
>>>    		eb->invalid_flags |= EXEC_OBJECT_NEEDS_GTT;
>>>    	return 0;
>>> diff --git a/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c b/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c
>>> index fc7fb33a3a52..947154e445a7 100644
>>> --- a/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c
>>> +++ b/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c
>>> @@ -704,7 +704,7 @@ static int igt_ctx_exec(void *arg)
>>>    				pr_err("Failed to fill dword %lu [%lu/%lu] with gpu (%s) [full-ppgtt? %s], err=%d\n",
>>>    				       ndwords, dw, max_dwords(obj),
>>>    				       engine->name,
>>> -				       yesno(!!rcu_access_pointer(ctx->vm)),
>>> +				       yesno(i915_gem_context_is_full_ppgtt(ctx)),
>>>    				       err);
>>>    				intel_context_put(ce);
>>>    				kernel_context_close(ctx);
>>> @@ -838,7 +838,7 @@ static int igt_shared_ctx_exec(void *arg)
>>>    				pr_err("Failed to fill dword %lu [%lu/%lu] with gpu (%s) [full-ppgtt? %s], err=%d\n",
>>>    				       ndwords, dw, max_dwords(obj),
>>>    				       engine->name,
>>> -				       yesno(!!rcu_access_pointer(ctx->vm)),
>>> +				       yesno(i915_gem_context_is_full_ppgtt(ctx)),
>>>    				       err);
>>>    				intel_context_put(ce);
>>>    				kernel_context_close(ctx);
>>> @@ -1417,7 +1417,7 @@ static int igt_ctx_readonly(void *arg)
>>>    				pr_err("Failed to fill dword %lu [%lu/%lu] with gpu (%s) [full-ppgtt? %s], err=%d\n",
>>>    				       ndwords, dw, max_dwords(obj),
>>>    				       ce->engine->name,
>>> -				       yesno(!!ctx_vm(ctx)),
>>> +				       yesno(i915_gem_context_is_full_ppgtt(ctx)),
>>>    				       err);
>>>    				i915_gem_context_unlock_engines(ctx);
>>>    				goto out_file;
>>>
>