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

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Date:   Tue, 29 Aug 2023 17:04:42 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Tomasz Figa <tfiga@...omium.org>, Anle Pan <anle.pan@....com>,
        Christoph Hellwig <hch@....de>, 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 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.