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

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Message-ID: <CAAFQd5CiHXvsJugSi+hXY9ESsmxUzBzmbhF6G48iVsOcL5eMtQ@mail.gmail.com>
Date:   Wed, 30 Aug 2023 12:47:57 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Christoph Hellwig <hch@....de>
Cc:     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 12:04 AM Christoph Hellwig <hch@....de> wrote:
>
> 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.

Do we see anything replacing it widely anywhere on the short-middle
term horizon? I think we could possibly migrate vb2 to use that new
thing internally and just provide some compatibility X to scatterlist
conversion function for the drivers.