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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4923fd30078sm274806595e9.7.2026.06.22.01.13.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 22 Jun 2026 01:13:45 -0700 (PDT) Date: Mon, 22 Jun 2026 09:13:44 +0100 From: David Laight To: David Hu Cc: Sumit Semwal , "Christian =?UTF-8?B?S8O2bmln?=" , Jason Gunthorpe , Nicolin Chen , Leon Romanovsky , Kevin Tian , Ankit Agrawal , Alex Williamson , linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, linux-kernel@vger.kernel.org, iommu@lists.linux.dev, jmoroni@google.com, praan@google.com, kpberry@google.com, sashiko-bot , stable@vger.kernel.org Subject: Re: [PATCH] dma-buf: Split sgl by largest page-aligned chunk Message-ID: <20260622091344.794e0d74@pumpkin> In-Reply-To: <20260621222130.1667453-1-xuehaohu@google.com> References: <20260621222130.1667453-1-xuehaohu@google.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: iommu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Sun, 21 Jun 2026 22:21:30 +0000 David Hu wrote: > Currently, `fill_sg_entry()` splits the scatterlist using `UINT_MAX`. > This creates a non-page-aligned DMA length (`0xFFFFFFFF`) for the > first entry, resulting in non-page-aligned DMA addresses for all > subsequent entries. How did you find this? It requires a single buffer over 4GB - seems highly unlikely. > > While the underlying IOMMU mapping may be contiguous, hardware > DMA engines often require explicit address alignment (e.g., page, > cacheline, or storage sector boundaries). Passing unaligned > addresses and lengths can cause explicit failures in DMA descriptor > creation or silent data corruption if lower unaligned bits are > truncated. > > Fix this by splitting the scatterlist by the largest possible page > aligned chunk within `UINT_MAX` (`ALIGN_DOWN(UINT_MAX, PAGE_SIZE)`). > This ensures all scatterlist DMA addresses and lengths remain page > aligned and satisfy hardware constraints. It would almost certainly better to spilt into 2G chunks. That removes any need for any divisions. > Page-aligned entries allow the system to cleanly chunk payloads into > PCIe MaxPayloadSize (MPS) (e.g., 128 bytes, 256 bytes, 512 bytes). > As a result, this may help reduce TLP fragmentation in P2P transfers > and alleviate potential congestion within a logical PCIe switch > partition, especially when Relaxed Ordering is not possible due to > hardware constraints. > > Reported-by: sashiko-bot > Closes: https://lore.kernel.org/all/20260609165431.778061F00893@smtp.kernel.org/ > Fixes: 3aa31a8bb11e ("dma-buf: provide phys_vec to scatter-gather mapping routine") > Cc: stable@vger.kernel.org > Signed-off-by: David Hu > --- > drivers/dma-buf/dma-buf-mapping.c | 13 ++++++++----- > 1 file changed, 8 insertions(+), 5 deletions(-) > > diff --git a/drivers/dma-buf/dma-buf-mapping.c b/drivers/dma-buf/dma-buf-mapping.c > index 794acff2546a..f2bde38fdb1f 100644 > --- a/drivers/dma-buf/dma-buf-mapping.c > +++ b/drivers/dma-buf/dma-buf-mapping.c > @@ -5,6 +5,9 @@ > */ > #include > #include > +#include > + > +#define MAX_ENT_SZ ALIGN_DOWN(UINT_MAX, PAGE_SIZE) > > static struct scatterlist *fill_sg_entry(struct scatterlist *sgl, size_t length, > dma_addr_t addr) > @@ -12,9 +15,9 @@ static struct scatterlist *fill_sg_entry(struct scatterlist *sgl, size_t length, > unsigned int len, nents; > int i; > > - nents = DIV_ROUND_UP(length, UINT_MAX); > + nents = DIV_ROUND_UP(length, MAX_ENT_SZ); > for (i = 0; i < nents; i++) { Why not change that to 'while (length) {' to avoid the division above. > - len = min_t(size_t, length, UINT_MAX); > + len = min_t(size_t, length, MAX_ENT_SZ); I bet that doesn't need to be min_t() > length -= len; > /* > * DMABUF abuses scatterlist to create a scatterlist > @@ -24,7 +27,7 @@ static struct scatterlist *fill_sg_entry(struct scatterlist *sgl, size_t length, > * does not require the CPU list for mapping or unmapping. > */ > sg_set_page(sgl, NULL, 0, 0); > - sg_dma_address(sgl) = addr + (dma_addr_t)i * UINT_MAX; > + sg_dma_address(sgl) = addr + (dma_addr_t)i * MAX_ENT_SZ; > sg_dma_len(sgl) = len; Replace the multiply with 'addr += len'. -- David > sgl = sg_next(sgl); > } > @@ -41,14 +44,14 @@ static unsigned int calc_sg_nents(struct dma_iova_state *state, > > if (!state || !dma_use_iova(state)) { > for (i = 0; i < nr_ranges; i++) > - nents += DIV_ROUND_UP(phys_vec[i].len, UINT_MAX); > + nents += DIV_ROUND_UP(phys_vec[i].len, MAX_ENT_SZ); > } else { > /* > * In IOVA case, there is only one SG entry which spans > * for whole IOVA address space, but we need to make sure > * that it fits sg->length, maybe we need more. > */ > - nents = DIV_ROUND_UP(size, UINT_MAX); > + nents = DIV_ROUND_UP(size, MAX_ENT_SZ); > } > > return nents;