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Thu, 25 Jan 2024 09:45:39 -0600 Received: from [10.249.42.149] ([10.249.42.149]) by lelvsmtp6.itg.ti.com (8.15.2/8.15.2) with ESMTP id 40PFjdK7110154; Thu, 25 Jan 2024 09:45:39 -0600 Message-ID: <7872f641-8a72-424f-b345-99c27403d7c6@ti.com> Date: Thu, 25 Jan 2024 09:45:39 -0600 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/3] udmabuf: Sync buffer mappings for attached devices Content-Language: en-US To: "Kasireddy, Vivek" , Gerd Hoffmann , Sumit Semwal , =?UTF-8?Q?Christian_K=C3=B6nig?= , Paul Cercueil CC: "linaro-mm-sig@lists.linaro.org" , "linux-kernel@vger.kernel.org" , "dri-devel@lists.freedesktop.org" , "linux-media@vger.kernel.org" References: <20240123221227.868341-1-afd@ti.com> <20240123221227.868341-2-afd@ti.com> From: Andrew Davis In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-EXCLAIMER-MD-CONFIG: e1e8a2fd-e40a-4ac6-ac9b-f7e9cc9ee180 On 1/24/24 5:05 PM, Kasireddy, Vivek wrote: > Hi Andrew, > >> Currently this driver creates a SGT table using the CPU as the >> target device, then performs the dma_sync operations against >> that SGT. This is backwards to how DMA-BUFs are supposed to behave. >> This may have worked for the case where these buffers were given >> only back to the same CPU that produced them as in the QEMU case. >> And only then because the original author had the dma_sync >> operations also backwards, syncing for the "device" on begin_cpu. >> This was noticed and "fixed" in this patch[0]. >> >> That then meant we were sync'ing from the CPU to the CPU using >> a pseudo-device "miscdevice". Which then caused another issue >> due to the miscdevice not having a proper DMA mask (and why should >> it, the CPU is not a DMA device). The fix for that was an even >> more egregious hack[1] that declares the CPU is coherent with >> itself and can access its own memory space.. >> >> Unwind all this and perform the correct action by doing the dma_sync >> operations for each device currently attached to the backing buffer. > Makes sense. > >> >> [0] commit 1ffe09590121 ("udmabuf: fix dma-buf cpu access") >> [1] commit 9e9fa6a9198b ("udmabuf: Set the DMA mask for the udmabuf >> device (v2)") >> >> Signed-off-by: Andrew Davis >> --- >> drivers/dma-buf/udmabuf.c | 41 +++++++++++++++------------------------ >> 1 file changed, 16 insertions(+), 25 deletions(-) >> >> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c >> index 3a23f0a7d112a..ab6764322523c 100644 >> --- a/drivers/dma-buf/udmabuf.c >> +++ b/drivers/dma-buf/udmabuf.c >> @@ -26,8 +26,6 @@ MODULE_PARM_DESC(size_limit_mb, "Max size of a >> dmabuf, in megabytes. Default is >> struct udmabuf { >> pgoff_t pagecount; >> struct page **pages; >> - struct sg_table *sg; >> - struct miscdevice *device; >> struct list_head attachments; >> struct mutex lock; >> }; >> @@ -169,12 +167,8 @@ static void unmap_udmabuf(struct >> dma_buf_attachment *at, >> static void release_udmabuf(struct dma_buf *buf) >> { >> struct udmabuf *ubuf = buf->priv; >> - struct device *dev = ubuf->device->this_device; >> pgoff_t pg; >> >> - if (ubuf->sg) >> - put_sg_table(dev, ubuf->sg, DMA_BIDIRECTIONAL); > What happens if the last importer maps the dmabuf but erroneously > closes it immediately? Would unmap somehow get called in this case? > Good question, had to scan the framework code a bit here. I thought closing a DMABUF handle would automatically unwind any current attachments/mappings, but it seems nothing in the framework does that. Looks like that is up to the importing drivers[0]: > Once a driver is done with a shared buffer it needs to call > dma_buf_detach() (after cleaning up any mappings) and then > release the reference acquired with dma_buf_get() by > calling dma_buf_put(). So closing a DMABUF after mapping without first unmapping it would be a bug in the importer, it is not the exporters problem to check for (although some more warnings in the framework checking for that might not be a bad idea..). Andrew [0] https://www.kernel.org/doc/html/v6.7/driver-api/dma-buf.html > Thanks, > Vivek > >> - >> for (pg = 0; pg < ubuf->pagecount; pg++) >> put_page(ubuf->pages[pg]); >> kfree(ubuf->pages); >> @@ -185,33 +179,31 @@ static int begin_cpu_udmabuf(struct dma_buf >> *buf, >> enum dma_data_direction direction) >> { >> struct udmabuf *ubuf = buf->priv; >> - struct device *dev = ubuf->device->this_device; >> - int ret = 0; >> - >> - if (!ubuf->sg) { >> - ubuf->sg = get_sg_table(dev, buf, direction); >> - if (IS_ERR(ubuf->sg)) { >> - ret = PTR_ERR(ubuf->sg); >> - ubuf->sg = NULL; >> - } >> - } else { >> - dma_sync_sg_for_cpu(dev, ubuf->sg->sgl, ubuf->sg->nents, >> - direction); >> - } >> + struct udmabuf_attachment *a; >> >> - return ret; >> + mutex_lock(&ubuf->lock); >> + >> + list_for_each_entry(a, &ubuf->attachments, list) >> + dma_sync_sgtable_for_cpu(a->dev, a->table, direction); >> + >> + mutex_unlock(&ubuf->lock); >> + >> + return 0; >> } >> >> static int end_cpu_udmabuf(struct dma_buf *buf, >> enum dma_data_direction direction) >> { >> struct udmabuf *ubuf = buf->priv; >> - struct device *dev = ubuf->device->this_device; >> + struct udmabuf_attachment *a; >> >> - if (!ubuf->sg) >> - return -EINVAL; >> + mutex_lock(&ubuf->lock); >> + >> + list_for_each_entry(a, &ubuf->attachments, list) >> + dma_sync_sgtable_for_device(a->dev, a->table, direction); >> + >> + mutex_unlock(&ubuf->lock); >> >> - dma_sync_sg_for_device(dev, ubuf->sg->sgl, ubuf->sg->nents, >> direction); >> return 0; >> } >> >> @@ -307,7 +299,6 @@ static long udmabuf_create(struct miscdevice >> *device, >> exp_info.priv = ubuf; >> exp_info.flags = O_RDWR; >> >> - ubuf->device = device; >> buf = dma_buf_export(&exp_info); >> if (IS_ERR(buf)) { >> ret = PTR_ERR(buf); >> -- >> 2.39.2 >