Re: [EXT] Re: [PATCH] media: videobuf2-dma-sg: limit the sg segment size

[Robin Murphy]

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@xxxxxx>
> Sent: Tuesday, August 29, 2023 11:04 PM
> To: Robin Murphy <robin.murphy@xxxxxxx>
> Cc: Tomasz Figa <tfiga@xxxxxxxxxxxx>; Anle Pan <anle.pan@xxxxxxx>; Christoph Hellwig <hch@xxxxxx>; m.szyprowski@xxxxxxxxxxx <m.szyprowski@xxxxxxxxxxx>; mchehab@xxxxxxxxxx <mchehab@xxxxxxxxxx>; linux-media@xxxxxxxxxxxxxxx <linux-media@xxxxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx <linux-kernel@xxxxxxxxxxxxxxx>; Hui Fang <hui.fang@xxxxxxx>
> Subject: [EXT] Re: [PATCH] media: videobuf2-dma-sg: limit the sg segment size
>
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>
> 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.