i915 dma_map_sg return value

[Chris Wilson]

On Wed, May 06, 2015 at 10:24:47AM +0200, Daniel Vetter wrote:
> On Tue, May 05, 2015 at 07:19:57PM +0200, Volker Vogelhuber wrote:
> > On 05.05.2015 17:51, Daniel Vetter wrote:
> > >On Tue, May 05, 2015 at 09:42:44AM +0200, Volker Vogelhuber wrote:
> > >>The documentation of the DMA-API writes the following about
> > >>dma_map_sg:
> > >>
> > >>"The implementation is free to merge several consecutive sglist entries
> > >>into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
> > >>consecutive sglist entries can be merged into one provided the first one
> > >>ends and the second one starts on a page boundary - in fact this is a huge
> > >>advantage for cards which either cannot do scatter-gather or have very
> > >>limited number of scatter-gather entries) and returns the actual number
> > >>of sg entries it mapped them to."
> > >>
> > >>I wonder why the return value of dma_map_sg is not returned in any way
> > >>from i915_gem_map_dma_buf. It only uses the return value for error
> > >>checking.
> > >>Can one be sure that in case of the i915 the nents value of the scatter
> > >>gather table is always equal to the value returned by dma_map_sg?
> > >>I'm asking because I want to use the sg table returned by
> > >>i915_gem_map_dma_buf in my own kernel module and iterate over it
> > >>using for_each_sg. And the example in the documentation of the DMA-API
> > >>uses the return value of dma_map_sg when calling for_each_sg and not
> > >>nents and it explicitly mentions:
> > >>
> > >>"Then you should loop count times (note: this can be less than nents times)"
> > >Hm, not looking at the return value of dma_map_sg is also how we use it
> > >internally in i915_gem_gtt.c. Not sure why we get away with this ...
> > Maybe you can be sure that on systems where the i915 driver is
> > used no reduction of the nents will be done by dma_map_sg?
> 
> Not sure that's true. But we do use for_each_sg_page, which might be
> immune to coallescing issue. But tbh I have no idea.

It is. nents is entirely redundant in the loop since we terminate with
the sentinel marker in the sg and iterate over amalgamated sg entries.

However, we probably should make sure that nents stays in sync in case
some other routine depends on its correctness.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre