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

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Message-ID: <20230831123532.GA11156@lst.de>
Date:   Thu, 31 Aug 2023 14:35:32 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Christoph Hellwig <hch@....de>, Tomasz Figa <tfiga@...omium.org>,
        Robin Murphy <robin.murphy@....com>,
        Anle Pan <anle.pan@....com>, m.szyprowski@...sung.com,
        mchehab@...nel.org, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org, hui.fang@....com
Subject: Re: [PATCH] media: videobuf2-dma-sg: limit the sg segment size

On Wed, Aug 30, 2023 at 01:43:57PM -0300, Jason Gunthorpe wrote:
> > > conversion function for the drivers.
> > 
> > Jason said at LSF/MM that he had a prototype for a mapping API that
> > takes a phys/len array as input and dma_addr/len a output, which really
> > is the right thing to do, especially for dmabuf.
> 
> Yes, still a prototype. Given the change in direction some of the
> assumptions of the list design will need some adjusting.
> 
> I felt there wasn't much justification to add a list without also
> supporting the P2P and it was not looking very good to give the DMA
> API proper p2p support without also connecting it to lists somehow.
> 
> Anyhow, I had drafted a basic list datastructure and starting
> implementation that is sort of structured in away that is similar to
> xarray (eg with fixed chunks, generic purpose, etc)
> 
> https://github.com/jgunthorpe/linux/commit/58d7e0578a09d9cd2360be515208bcd74ade5958

This seems fairly complicated complicated, and the entry seems pretty large
for a bio_vec replacement or a dma_addr_t+len tuple, which both should
be (sizeof(phys_addr_t) + sizeof(u32) + the size of flags if needed, which
for 64-bit would fit into the padding from 96 bytes to 128 bytes anyway.