On Tue, 2019-03-12 at 08:08 +0100, Stanislaw Gruszka wrote: > On Mon, Mar 11, 2019 at 08:47:44AM -0700, Alexander Duyck wrote: > > > drivers/iommu/amd_iommu.c | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/drivers/iommu/amd_iommu.c b/drivers/iommu/amd_iommu.c > > > index 6b0760dafb3e..949621f33624 100644 > > > --- a/drivers/iommu/amd_iommu.c > > > +++ b/drivers/iommu/amd_iommu.c > > > @@ -2604,7 +2604,7 @@ static int map_sg(struct device *dev, struct scatterlist *sglist, > > > > > > /* Everything is mapped - write the right values into s->dma_address */ > > > for_each_sg(sglist, s, nelems, i) { > > > - s->dma_address += address + s->offset; > > > + s->dma_address += address + (s->offset & ~PAGE_MASK); > > > s->dma_length = s->length; > > > } > > > > > > > You should add a comment calling out that this is needed because the > > sg_phys(s) call above this is masked with PAGE_MASK. Then this makes > > much more sense. Otherwise I would have assumed you needed either the > > full offset or none. > > Would something like this > > /* > * Everything is mapped - write the right values into s->dma_address. > * Take into account s->offset can be bigger than page size and sg_phys(s) > * address has to be aligned to page granularity. > */ > > be appropriate ? > > Stanislaw > No, that isn't a good description. If you take a look at the code a few lines up you find: phys_addr = (sg_phys(s) & PAGE_MASK) + (j << PAGE_SHIFT); Now if I am not mistaken the whole reason why you are having to make the change here is because the application of PAGE_MASK in this line. Basically what sg_phys() will do is take the address of the page, convert it to a physical address and add the offset. However what the mask is doing is limiting how much of that offset can be added. As a result you have to add the remainder that was masked out. So maybe a better comment would be something like: /* * Add in the remaining piece of the scatter-gather offset that was * masked out when we were determining the physical address via * (sg_phys(s) & PAGE_MASK) earlier. */