Re: [PATCH 13/18] drm/i915: Introduce i915_gem_object_create_stolen()

[Chris Wilson <chris@chris-wilson.co.uk>]

On Mon, 5 Nov 2012 16:32:26 +0000, Ben Widawsky <ben@bwidawsk.net> wrote:
> On Fri, 19 Oct 2012 18:03:19 +0100
> Chris Wilson <chris@chris-wilson.co.uk> wrote:
> 
> > Allow for the creation of GEM objects backed by stolen memory. As these
> > are not backed by ordinary pages, we create a fake dma mapping and store
> > the address in the scatterlist rather than obj->pages.
> > 
> > v2: Mark _i915_gem_object_create_stolen() as static, as noticed by Jesse
> > Barnes.
> > 
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
> 
> Deferring on an r-b for now until I understand the point of most of this
> patch.

The stolen support is a precursor for fastboot, where we need to wrap
the allocations made by the BIOS from the stolen memory and reuse that
for our own framebuffers.
 
> > +	struct scatterlist *sg;
> > +
> 
> BUG_ON(offset + size <= dev_priv->mm.gtt->stolen_size);

Done with a minor amendment.
 
> > +	/* We hide that we have no struct page backing our stolen object
> > +	 * by wrapping the contiguous physical allocation with a fake
> > +	 * dma mapping in a single scatterlist.
> > +	 */
> > +
> > +	st = kmalloc(sizeof(*st), GFP_KERNEL);
> > +	if (st == NULL)
> > +		return NULL;
> > +
> > +	if (!sg_alloc_table(st, 1, GFP_KERNEL)) {
Fixed.

> > +		kfree(st);
> > +		return NULL;
> > +	}
> > +
> > +	sg = st->sgl;
> > +	sg->offset = offset;
> > +	sg->length = size;
> > +
> > +	sg_dma_address(sg) = dev_priv->mm.stolen_base + offset;
> > +	sg_dma_len(sg) = size;
> > +
> 
> Do we want to make stolen_base a dma_addr_t (or at least typecast it)?

Interesting enough, the current FBC registers are limited to only using
32bit addresses, so stolen_base atm is not technically a dma_addr_t.
Maybe I'm picking hairs. :)
 
> > +	return st;
> > +}
> > +
> > +static int i915_gem_object_get_pages_stolen(struct drm_i915_gem_object *obj)
> > +{
> > +	BUG();
> > +	return -EINVAL;
> > +}
> > +
> 
> __noreturn, or maybe just make .get_pages = NULL, and do the check in
> the upper layer get_pages?

I refer you to http://lwn.net/Articles/336262/ where the argument is put
forth that default no-op functions are preferrable in most cases to
interpretting special NULL vfuncs. We have adopted this elsewhere in
i915.ko to good effect.

> > +	stolen = drm_mm_search_free(&dev_priv->mm.stolen, size, 4096, 0);
> > +	if (stolen)
> > +		stolen = drm_mm_get_block(stolen, size, 4096);
> > +	if (stolen == NULL)
> > +		return NULL;
> 
> Could probably do slightly better here with ERR_PTR(-ENOMEM) but since
> we don't do that elsewhere, I guess it doesn't matter.

I was tempted - it would have just looked odd as being the only create
routine to do so. :)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre