On Sun, Mar 3, 2019 at 11:10 PM Stanislaw Gruszka <sgruszka@xxxxxxxxxx> wrote: > > On Thu, Feb 28, 2019 at 02:40:29PM +0100, Joerg Roedel wrote: > > On Thu, Feb 28, 2019 at 01:19:48PM +0100, Stanislaw Gruszka wrote: > > > Nevermind, the patch is wrong, s->dma_address is initalized in sg_num_pages(). > > > > Yes, it is. In sg_num_pages() the offset into the IOMMU mapping is > > stored in s->dma_address, taking also the segment boundary mask into > > account. map_sg() later only adds the base-address to that. > > I have some more info about the issues in > https://bugzilla.kernel.org/show_bug.cgi?id=202673 > > We have some bugs in mt76. Apparently we should not use > page_frag_alloc() with size bigger than PAGE_SIZE as page_frag_alloc() > can fallback to single page allocation. And also we should not make > sizes unaligned as pointed in commit: > 3bed3cc4156e ("net: Do not allocate page fragments that are not skb aligned" As a small and totally unrelated note, page_frag_alloc is only used in mt76 and the nvme driver ;) > > However after fixing that mt76usb still did not work. To make things > work we had to change rx frag size from 2048 to PAGE_SIZE and change > virt_to_head_page() to virt_to_page() when setting SG's. > > I think I understand why first change was needed. If we do 2 separate > dma maps of 2 different buffers in single page i.e (PAGE + off=0 > and PAGE + off=2048) it causes problem. So either map_sg() return > error which mt76usb does not handle correctly or there is issue > in AMD IOMMU because two dma maps use the same page. > > But I don't understand why the second change was needed. Without > it we have issue with incorrect page->_refcount . It is somehow > related with AMD IOMMU, because on different platforms we do not > have such problems. > > Joerg, could you look at this ? Thanks. > > Stanislaw
