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 ... -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
