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. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
