Re: [PATCH 1/6] drm/i915: Clearing buffer objects via CPU/GTT

[Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>]

On Wed, 2015-12-09 at 13:26 +0000, Dave Gordon wrote:
> On 09/12/15 12:46, ankitprasad.r.sharma@intel.com wrote:
> > From: Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>
> >
> > This patch adds support for clearing buffer objects via CPU/GTT. This
> > is particularly useful for clearing out the non shmem backed objects.
> > Currently intend to use this only for buffers allocated from stolen
> > region.
> >
> > v2: Added kernel doc for i915_gem_clear_object(), corrected/removed
> > variable assignments (Tvrtko)
> >
> > v3: Map object page by page to the gtt if the pinning of the whole object
> > to the ggtt fails, Corrected function name (Chris)
> >
> > Testcase: igt/gem_stolen
> >
> > Signed-off-by: Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>
> > Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> > ---
> >   drivers/gpu/drm/i915/i915_drv.h |  1 +
> >   drivers/gpu/drm/i915/i915_gem.c | 79 +++++++++++++++++++++++++++++++++++++++++
> >   2 files changed, 80 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> > index 548a0eb..8e554d3 100644
> > --- a/drivers/gpu/drm/i915/i915_drv.h
> > +++ b/drivers/gpu/drm/i915/i915_drv.h
> > @@ -2856,6 +2856,7 @@ int i915_gem_obj_prepare_shmem_read(struct drm_i915_gem_object *obj,
> >   				    int *needs_clflush);
> >
> >   int __must_check i915_gem_object_get_pages(struct drm_i915_gem_object *obj);
> > +int i915_gem_object_clear(struct drm_i915_gem_object *obj);
> >
> >   static inline int __sg_page_count(struct scatterlist *sg)
> >   {
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index 9d2e6e3..d57e850 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -5244,3 +5244,82 @@ fail:
> >   	drm_gem_object_unreference(&obj->base);
> >   	return ERR_PTR(ret);
> >   }
> > +
> > +/**
> > + * i915_gem_clear_object() - Clear buffer object via CPU/GTT
> > + * @obj: Buffer object to be cleared
> > + *
> > + * Return: 0 - success, non-zero - failure
> > + */
> > +int i915_gem_object_clear(struct drm_i915_gem_object *obj)
> > +{
> > +	int ret, i;
> > +	char __iomem *base;
> > +	size_t size = obj->base.size;
> > +	struct drm_i915_private *i915 = to_i915(obj->base.dev);
> > +	struct drm_mm_node node;
> > +
> > +	WARN_ON(!mutex_is_locked(&obj->base.dev->struct_mutex));
> > +	ret = i915_gem_obj_ggtt_pin(obj, 0, PIN_MAPPABLE | PIN_NONBLOCK);
> > +	if (ret) {
> > +		memset(&node, 0, sizeof(node));
> > +		ret = drm_mm_insert_node_in_range_generic(&i915->gtt.base.mm,
> > +							  &node, 4096, 0,
> > +							  I915_CACHE_NONE, 0,
> > +							  i915->gtt.mappable_end,
> > +							  DRM_MM_SEARCH_DEFAULT,
> > +							  DRM_MM_CREATE_DEFAULT);
> > +		if (ret)
> > +			goto out;
> > +
> > +		i915_gem_object_pin_pages(obj);
> > +	} else {
> > +		node.start = i915_gem_obj_ggtt_offset(obj);
> > +		node.allocated = false;
> > +	}
> > +
> > +	ret = i915_gem_object_put_fence(obj);
> > +	if (ret)
> > +		goto unpin;
> > +
> > +	if (node.allocated) {
> > +		for (i = 0; i < size/PAGE_SIZE; i++) {
> > +			wmb();
> > +			i915->gtt.base.insert_page(&i915->gtt.base,
> > +					i915_gem_object_get_dma_address(obj, i),
> > +					node.start,
> > +					I915_CACHE_NONE,
> > +					0);
> > +			wmb();
> > +			base = ioremap_wc(i915->gtt.mappable_base + node.start, 4096);
> > +			memset_io(base, 0, 4096);
> > +			iounmap(base);
> > +		}
> > +	} else {
> > +		/* Get the CPU virtual address of the buffer */
> > +		base = ioremap_wc(i915->gtt.mappable_base +
> > +				  node.start, size);
> > +		if (base == NULL) {
> > +			DRM_ERROR("Mapping of gem object to CPU failed!\n");
> > +			ret = -ENOSPC;
> > +			goto unpin;
> > +		}
> > +
> > +		memset_io(base, 0, size);
> > +		iounmap(base);
> > +	}
> > +unpin:
> > +	if (node.allocated) {
> > +		wmb();
> > +		i915->gtt.base.clear_range(&i915->gtt.base,
> > +				node.start, node.size,
> > +				true);
> > +		drm_mm_remove_node(&node);
> > +		i915_gem_object_unpin_pages(obj);
> > +	}
> > +	else {
> > +		i915_gem_object_ggtt_unpin(obj);
> > +	}
> > +out:
> > +	return ret;
> > +}
> 
> This is effectively two functions interleaved, as shown by the repeated 
> if (node.allocated) tests. Would it not be clearer to have the mainline 
> function deal only with the GTT-pinned case, and a separate function for 
> the page-by-page version, called as a fallback if pinning fails?
> 
> int i915_gem_object_clear(struct drm_i915_gem_object *obj)
> {
> 	int ret, i;
> 	char __iomem *base;
> 	size_t size = obj->base.size;
> 	struct drm_i915_private *i915 = to_i915(obj->base.dev);
> 
> 	WARN_ON(!mutex_is_locked(&obj->base.dev->struct_mutex));
> 	ret = i915_gem_obj_ggtt_pin(obj, 0, PIN_MAPPABLE|PIN_NONBLOCK);
> 	if (ret)
> 		return __i915_obj_clear_by_pages(...);
> 
> 	... mainline (fast) code here ...
> 
> 	return ret;
> }
> 
> static int __i915_obj_clear_by_pages(...);
> {
> 	... complicated page-by-page fallback code here ...
> }
> 
This is good to separate the page-by-page path to not make the code
messy, Also I kind of liked Chris' suggestion to not use ioremap_wc() as
it could easily exhaust kernel space.

To make it less messy and more robust, I would prefer to use only the
page-by-page path (no need to even try mapping the full object), with
io_mapping_map_wc()

Thanks,
Ankit
> 


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