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[156.34.48.30]) by smtp.gmail.com with ESMTPSA id n11sm5205534qkk.105.2020.10.04.05.51.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 04 Oct 2020 05:51:00 -0700 (PDT) Received: from jgg by mlx with local (Exim 4.94) (envelope-from ) id 1kP3Tf-007HEC-QI; Sun, 04 Oct 2020 09:50:59 -0300 Date: Sun, 4 Oct 2020 09:50:59 -0300 From: Jason Gunthorpe To: Daniel Vetter Cc: DRI Development , LKML , Daniel Vetter , Andrew Morton , John Hubbard , =?utf-8?B?SsOpcsO0bWU=?= Glisse , Jan Kara , Dan Williams , Linux MM , Linux ARM , Pawel Osciak , Marek Szyprowski , Kyungmin Park , Tomasz Figa , Inki Dae , Joonyoung Shim , Seung-Woo Kim , linux-samsung-soc , "open list:DMA BUFFER SHARING FRAMEWORK" , Oded Gabbay Subject: Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM Message-ID: <20201004125059.GP9916@ziepe.ca> References: <20201002175303.390363-1-daniel.vetter@ffwll.ch> <20201002175303.390363-2-daniel.vetter@ffwll.ch> <20201002180603.GL9916@ziepe.ca> <20201002233118.GM9916@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org On Sat, Oct 03, 2020 at 11:40:22AM +0200, Daniel Vetter wrote: > > That leaves the only interesting places as vb2_dc_get_userptr() and > > vb2_vmalloc_get_userptr() which both completely fail to follow the > > REQUIRED behavior in the function's comment about checking PTEs. It > > just DMA maps them. Badly broken. > > > > Guessing this hackery is for some embedded P2P DMA transfer? > > Yeah, see also the follow_pfn trickery in > videobuf_dma_contig_user_get(), I think this is fully intentional and > userspace abi we can't break :-/ We don't need to break uABI, it just needs to work properly in the kernel: vma = find_vma_intersection() dma_buf = dma_buf_get_from_vma(vma) sg = dma_buf_p2p_dma_map(dma_buf) [.. do dma ..] dma_buf_unmap(sg) dma_buf_put(dma_buf) It is as we discussed before, dma buf needs to be discoverable from a VMA, at least for users doing this kind of stuff. > Yup this should be done with dma_buf instead, and v4l has that. But > old uapi and all that. This is why I said we might need a new > VM_DYNAMIC_PFNMAP or so, to make follow_pfn not resolve this in the > case where the driver manages the underlying iomem range (or whatever > it is) dynamically and moves buffer objects around, like drm drivers > do. But I looked, and we've run out of vma->vm_flags :-( A VM flag doesn't help - we need to introduce some kind of lifetime, and that has to be derived from the VMA. It needs data not just a flag > The other problem is that I also have no real working clue about all > the VM_* flags and what they all mean, and whether drm drivers set the > right ones in all cases (they probably don't, but oh well). > Documentation for this stuff in headers is a bit thin at times. Yah, I don't really know either :\ The comment above vm_normal_page() is a bit helpful. Don't know what VM_IO/VM_PFNMAP mean in their 3 combinations There are very few places that set VM_PFNMAP without VM_IO.. Jason