Re: [PATCH 2/2] drm/i915: Drop i915_gem_obj_is_pinned() from set-cache-level

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

On 08/10/15 10:46, Chris Wilson wrote:
> On Thu, Oct 08, 2015 at 10:32:39AM +0100, Tvrtko Ursulin wrote:
>>
>> On 07/10/15 17:19, Chris Wilson wrote:
>>> On Wed, Oct 07, 2015 at 04:57:25PM +0100, Tvrtko Ursulin wrote:
>>>>
>>>> Hi,
>>>>
>>>> On 06/10/15 11:39, Chris Wilson wrote:
>>>>> Since the remove of the pin-ioctl, we only care about not changing the
>>>>> cache level on buffers pinned to the hardware as indicated by
>>>>> obj->pin_display. So we can safely replace i915_gem_object_is_pinned()
>>
>> [snip]
>>
>>>>> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
>>>>> index d4a3bdf0c5b6..2b8ed7a2faab 100644
>>>>> --- a/drivers/gpu/drm/i915/i915_gem.c
>>>>> +++ b/drivers/gpu/drm/i915/i915_gem.c
>>>>> @@ -3629,31 +3629,34 @@ int i915_gem_object_set_cache_level(struct drm_i915_gem_object *obj,
>>>>>   {
>>>>>   	struct drm_device *dev = obj->base.dev;
>>>>>   	struct i915_vma *vma, *next;
>>>>> +	bool bound = false;
>>>>>   	int ret = 0;
>>>>>
>>>>>   	if (obj->cache_level == cache_level)
>>>>>   		goto out;
>>>>>
>>>>> -	if (i915_gem_obj_is_pinned(obj)) {
>>>>> -		DRM_DEBUG("can not change the cache level of pinned objects\n");
>>>>> -		return -EBUSY;
>>>>> -	}
>>>>> -
>>>>>   	list_for_each_entry_safe(vma, next, &obj->vma_list, vma_link) {
>>>>> +		if (!drm_mm_node_allocated(&vma->node))
>>>>> +			continue;
>>>>> +
>>>>> +		if (vma->pin_count) {
>>>>> +			DRM_DEBUG("can not change the cache level of pinned objects\n");
>>>>> +			return -EBUSY;
>>>>> +		}
>>>>> +
>>>>
>>>> But this is the same as i915_gem_obj_is_pinned, where is the
>>>> obj->pin_display change commit message talks about?
>>>
>>> Right here. The difference is that we are only iterating the vma list
>>> once rather than 3x.
>>
>> Thats true, but the commit says it is going to use obj->pin_display
>> for something and then doesn't use it at all. Riddles in patches are
>> not that hot. :)
>
> I was trying to explain what the actual rules pertaining to the
> rebinding the vma was. We can rebind anything that isn't pinned and the
> only thing pinned here can be obj->pin_display.

Okay but the commit message says the is going to use obj->pin_display.

>>>>>   		if (!i915_gem_valid_gtt_space(vma, cache_level)) {
>>>>>   			ret = i915_vma_unbind(vma);
>>>>>   			if (ret)
>>>>>   				return ret;
>>>>> -		}
>>>>> +		} else
>>>>> +			bound = true;
>>>>>   	}
>>>>>
>>>>> -	if (i915_gem_obj_bound_any(obj)) {
>>>>> +	if (bound) {
>>>>>   		ret = i915_gem_object_wait_rendering(obj, false);
>>>>>   		if (ret)
>>>>>   			return ret;
>>>>>
>>>>> -		i915_gem_object_finish_gtt(obj);
>>>>> -
>>>>>   		/* Before SandyBridge, you could not use tiling or fence
>>>>>   		 * registers with snooped memory, so relinquish any fences
>>>>>   		 * currently pointing to our region in the aperture.
>>>>> @@ -3664,13 +3667,18 @@ int i915_gem_object_set_cache_level(struct drm_i915_gem_object *obj,
>>>>>   				return ret;
>>>>>   		}
>>>>>
>>>>> -		list_for_each_entry(vma, &obj->vma_list, vma_link)
>>>>> -			if (drm_mm_node_allocated(&vma->node)) {
>>>>> -				ret = i915_vma_bind(vma, cache_level,
>>>>> -						    PIN_UPDATE);
>>>>> -				if (ret)
>>>>> -					return ret;
>>>>> -			}
>>>>> +		/* Access to snoopable pages through the GTT is incoherent. */
>>>>> +		if (cache_level != I915_CACHE_NONE && !HAS_LLC(dev))
>>>>> +			i915_gem_release_mmap(obj);
>>>>
>>>> Don't fully understand this one - but my question is this.
>>>> Previously userspace would lose mappings on cache level changes any
>>>> time, after this only on !LLC when turning on caching mode. So this
>>>> means userspace needs to know about this change and modify it's
>>>> behavior? Or what exactly would happen in practice?
>>>
>>> No. Userspace has no knowledge of the kernel handling the PTEs, its
>>> mapping is persistent (i.e. the obj->mmap_offset inside the dev->mappping).
>>> Otoh, we are improving the situation so even if userspace tries to avoid
>>> set-cache-level nothing is lost.
>>
>> Hm so if a VMA is re-bound in this process and it could have gotten
>> a new GGTT address, why it is not necessary to always release mmaps
>> and so to update CPU PTEs?
>
> The VMA are not moved by this function, only the PTEs are rewritten. The
> GTT ignores the bits we are changing on llc architectures. On !llc using
> the GTT to access snoopable PTE is verboten and does cause machine hangs.

How come they are not moved when they can be unbound and then bound again?

Regards,

Tvrtko
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