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

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Date:   Fri, 1 Sep 2023 08:10:14 +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 Thu, Aug 31, 2023 at 12:32:38PM -0300, Jason Gunthorpe wrote:
> The entry is variable sized, so it depends on what is stuffed in
> it. For alot of common use cases, especially RDMA page lists, it will
> be able to use an 8 byte entry. This is pretty much the most space
> efficient it could be.

How do you get away with a 8 byte entry for addr+len?

> The primary alternative I see is a fixed 16 bytes/entry with a 64 bit
> address and ~60 bit length + ~4 bits of flags. This is closer to bio,
> simpler and faster, but makes the RDMA cases 2x bigger.

That's what I'd expect.

> With your direction I felt we could safely keep bio as it is and
> cheaply make a fast DMA mapper for it. Provide something like this as
> the 'kitchen sink' version for dmabuf/rdma/etc that are a little
> different.

So for the first version I see no need to change the bio_vec
representation as part of this project, but at the same time the
bio_vec representation causes problems for other reasons.  So I want
to change it anyway.