linux-kernel - Re: [PATCH 1/3] dma-buf: Introduce dma buffer sharing mechanism

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Message-ID: <CAB2ybb_5Co38kpahe5PmO5b_uevr0=_3ODnwD1Xy9V0HQ7mPwg@mail.gmail.com>
Date:	Thu, 26 Jan 2012 11:05:30 +0530
From:	"Semwal, Sumit" <sumit.semwal@...com>
To:	Tomasz Stanislawski <t.stanislaws@...sung.com>,
	Sumit Semwal <sumit.semwal@...com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-mm@...ck.org, linaro-mm-sig@...ts.linaro.org,
	dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
	arnd@...db.de, airlied@...hat.com, linux@....linux.org.uk,
	jesse.barker@...aro.org, m.szyprowski@...sung.com, rob@...com,
	patches@...aro.org, Sumit Semwal <sumit.semwal@...aro.org>
Cc:	daniel@...ll.ch
Subject: Re: [PATCH 1/3] dma-buf: Introduce dma buffer sharing mechanism

On Thu, Jan 26, 2012 at 1:37 AM, Daniel Vetter <daniel@...ll.ch> wrote:
> On Wed, Jan 25, 2012 at 06:02:41PM +0100, Tomasz Stanislawski wrote:
>> Hi Sumit,
>>
>> On 12/26/2011 10:23 AM, Sumit Semwal wrote:
>> >This is the first step in defining a dma buffer sharing mechanism.
>> >
>> [snip]
>>
>> >+struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *,
>> >+                                    enum dma_data_direction);
>> >+void dma_buf_unmap_attachment(struct dma_buf_attachment *, struct sg_table *);
>>
>> I think that you should add enum dma_data_direction as an argument
>> unmap function. It was mentioned that the dma_buf_attachment should keep
>> cached and mapped sg_table for performance reasons. The field
>> dma_buf_attachment::priv seams to be a natural place to keep this sg_table.
>> To map a buffer the exporter calls dma_map_sg. It needs dma direction
>> as an argument. The problem is that dma_unmap_sg also needs this
>> argument but dma direction is not available neither in
>> dma_buf_unmap_attachment nor in unmap callback. Therefore the exporter
>> is forced to embed returned sg_table into a bigger structure where
>> dma direction is remembered. Refer to function vb2_dc_dmabuf_ops_map
>> at
>
> Oops, makes sense. I've totally overlooked that we need to pass in the dma
> direction also for the unmap call to the dma subsystem. Sumit, can you
> stitch together that small patch?

Right, of course. I will do that by tomorrow; it is a bank holiday
today here in India, so.

regards,
~Sumit.
> -Daniel
> --
> Daniel Vetter
> Mail: daniel@...ll.ch
> Mobile: +41 (0)79 365 57 48
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