linux-kernel - Re: [PATCH v2 1/3] media/videobuf2-dma-sg: Fix handling of sg

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Message-ID: <CAPybu_0wpNU0m2jjmbff+-mcoU-dkKjpHoW8Hr-GPyWH4oGcgQ@mail.gmail.com>
Date:	Fri, 13 Feb 2015 16:20:16 +0100
From:	Ricardo Ribalda Delgado <ricardo.ribalda@...il.com>
To:	Hans Verkuil <hverkuil@...all.nl>
Cc:	Pawel Osciak <pawel@...iak.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	linux-media <linux-media@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Hans Verkuil <hans.verkuil@...co.com>,
	martin.petersen@...cle.com, hch@....de, tonyb@...ernetics.com,
	axboe@...com, Stephen Rothwell <sfr@...b.auug.org.au>,
	lauraa@...eaurora.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	webbnh@...com, hare@...e.de,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2 1/3] media/videobuf2-dma-sg: Fix handling of sg_table structure

Hello Hans

On Fri, Feb 13, 2015 at 4:02 PM, Hans Verkuil <hverkuil@...all.nl> wrote:
> Hi Ricardo, Marek,
>
> I have a few questions, mostly to improve my own understanding.
>
> First of all, is this solving an actual bug for you, or did you just find
> it while reviewing code? And if it solves a bug, then which architecture
> are you using? ARM? Intel?
>

My arch is intel based (AMD APU). I found it while doing review. While
updating our kernel to 3.19 I had to patch some of my out of tree
drivers, and then I gave a look to the file.

>> dma_map_sg returns the number of areas mapped by the hardware,
>> which could be different than the areas given as an input.
>> The output must be saved to nent.
>>
>> The output of dma_map, should be used to transverse the scatter list.
>>
>> dma_unmap_sg needs the value passed to dma_map_sg (nents_orig).
>
> I noticed that few dma_unmap_sg calls actually use orig_nents. It makes
> me wonder if the dma_unmap_sg documentation is actually correct. It does
> clearly state that orig_nents should be used, and it might well be that
> the only reason this hasn't led to problems is that very few architectures
> actually seem to return nents < orig_nents.

It is not the most clear API to use :(. Some of the prototypes do not
make a lot of sense, and it is documented outside the code.

I have sent these two patches:

https://lkml.org/lkml/2015/2/11/231
https://lkml.org/lkml/2015/2/11/232

>> +     sgt->nents = dma_map_sg_attrs(buf->dev, sgt->sgl, sgt->orig_nents,
>> +                                   buf->dma_dir, &attrs);
>
> Is a driver free to change sgt->nents? It's unclear from the documentation
> or code that that is actually the purpose of sgt->nents. Most drivers seem
> to store the result of dma_map_sg into a driver-specific struct.

As I understand it, this is the purpose of the struct scatter list,
have at hand the three values that you need,
the sgl, nents and orig_ents.

But it would be great if the maintaner of the dma-api speaks up :)

I am putting  get_maintainer.pl in cc

Thanks Hans!
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