Messages in this thread | | | Date | Wed, 30 Aug 2023 14:41:34 +0100 | Subject | Re: [EXT] Re: [PATCH] media: videobuf2-dma-sg: limit the sg segment size | From | Robin Murphy <> |
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On 2023-08-30 04:22, Hui Fang wrote: > Hi, guys > Thanks for your time. > I wonder if only NXP met the "swiotlb buffer full" issue. > In theory, when format is YUYV, those resolutions no greater than 320x240 (153600 bytes, which less than the max mapping size 256K ) can't meet the issue. > But resolutions no less than 640x480 (307200 bytes), may have chances to trigger the issue. > On our side, we tested on 1080p, easy to reproduce. > Or maybe "dma_max_mapping_size(dev)" is much bigger than 256K on your side?
I would imagine that in most cases, people either have an IOMMU, or they don't have more RAM than the device can access directly.
And in fact I think we've missed looking deep enough and that's the real problem here - the overall context is a buffer allocator, so if SWIOTLB is bouncing anything in that dma_map_sg() call, then it means we're allocating a buffer for a device out of pages that that device can't actually access, which seems fundamentally bad. Unfortunately there isn't currently an easy way to do the right thing - dma-debug would probably get very unhappy about scraping a bunch of dma_alloc_pages() allocations into a scatterlist and subsequently calling dma_sync_sg() on it, while dma_alloc_noncontiguous() does at least return a scatterlist but would be overly restrictive since we don't need it to be dma-contiguous either. I guess we could do with some sort of dma_alloc_sgt() API that joins the relevant internal bits together to replace vb2_dma_sg_alloc_compacted() with proper DMA-mask-aware allocation, and probably also allocates the sg_table as well?
Thanks, Robin.
> > BRs, > Fang Hui > ________________________________ > From: Christoph Hellwig <hch@lst.de> > Sent: Tuesday, August 29, 2023 11:04 PM > To: Robin Murphy <robin.murphy@arm.com> > Cc: Tomasz Figa <tfiga@chromium.org>; Anle Pan <anle.pan@nxp.com>; Christoph Hellwig <hch@lst.de>; m.szyprowski@samsung.com <m.szyprowski@samsung.com>; mchehab@kernel.org <mchehab@kernel.org>; linux-media@vger.kernel.org <linux-media@vger.kernel.org>; linux-kernel@vger.kernel.org <linux-kernel@vger.kernel.org>; Hui Fang <hui.fang@nxp.com> > Subject: [EXT] Re: [PATCH] media: videobuf2-dma-sg: limit the sg segment size > > Caution: This is an external email. Please take care when clicking links or opening attachments. When in doubt, report the message using the 'Report this email' button > > > On Tue, Aug 29, 2023 at 12:14:44PM +0100, Robin Murphy wrote: >> dma_get_max_seg_size() represents a capability of the device itself, namely >> the largest contiguous range it can be programmed to access in a single DMA >> descriptor/register/whatever. > > Yes. In a way it's a bit odd that it ended up in a field in > struct device, as the feature might actually be different for different > DMA engines or features in a device. If I was to redesign it from > scratch I'd just pass it to dma_map_sg. > >>> Generally looking at videobuf2-dma-sg, I feel like we would benefit >>> from some kind of dma_alloc_table_from_pages() that simply takes the >>> struct dev pointer and does everything necessary. >> >> Possibly; this code already looks lifted from drm_prime_pages_to_sg(), and >> if it's needed here then presumably vb2_dma_sg_get_userptr() also needs it, >> at the very least. > > Yes, there's tons of them. But I'd feel really bad adding even more > struct scatterlist based APIs given how bad of a data structure that is. >
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