Hello Hans On Fri, Feb 13, 2015 at 4:02 PM, Hans Verkuil <hverkuil@xxxxxxxxx> wrote: > Hi Ricardo, Marek, > > I have a few questions, mostly to improve my own understanding. > > First of all, is this solving an actual bug for you, or did you just find > it while reviewing code? And if it solves a bug, then which architecture > are you using? ARM? Intel? > My arch is intel based (AMD APU). I found it while doing review. While updating our kernel to 3.19 I had to patch some of my out of tree drivers, and then I gave a look to the file. >> dma_map_sg returns the number of areas mapped by the hardware, >> which could be different than the areas given as an input. >> The output must be saved to nent. >> >> The output of dma_map, should be used to transverse the scatter list. >> >> dma_unmap_sg needs the value passed to dma_map_sg (nents_orig). > > I noticed that few dma_unmap_sg calls actually use orig_nents. It makes > me wonder if the dma_unmap_sg documentation is actually correct. It does > clearly state that orig_nents should be used, and it might well be that > the only reason this hasn't led to problems is that very few architectures > actually seem to return nents < orig_nents. It is not the most clear API to use :(. Some of the prototypes do not make a lot of sense, and it is documented outside the code. I have sent these two patches: https://lkml.org/lkml/2015/2/11/231 https://lkml.org/lkml/2015/2/11/232 >> + sgt->nents = dma_map_sg_attrs(buf->dev, sgt->sgl, sgt->orig_nents, >> + buf->dma_dir, &attrs); > > Is a driver free to change sgt->nents? It's unclear from the documentation > or code that that is actually the purpose of sgt->nents. Most drivers seem > to store the result of dma_map_sg into a driver-specific struct. As I understand it, this is the purpose of the struct scatter list, have at hand the three values that you need, the sgl, nents and orig_ents. But it would be great if the maintaner of the dma-api speaks up :) I am putting get_maintainer.pl in cc Thanks Hans! -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
