linux-kernel - Re: [PATCH] dma-buf: Expose buffer size to userspace (v2)

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Message-ID: <522727A4.6050109@free.fr>
Date:	Wed, 04 Sep 2013 14:29:24 +0200
From:	Martin Peres <martin.peres@...e.fr>
To:	Christopher James Halse Rogers 
	<christopher.halse.rogers@...onical.com>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
	robclark@...il.com, linux-media@...r.kernel.org
Subject: Re: [PATCH] dma-buf: Expose buffer size to userspace (v2)

Le 04/09/2013 14:05, Daniel Vetter a écrit :
> On Wed, Sep 04, 2013 at 12:24:27PM +0200, Martin Peres wrote:
>> Hi Christopher,
>> Le 04/09/2013 05:15, Christopher James Halse Rogers a écrit :
>>> Each dma-buf has an associated size and it's reasonable for userspace
>>> to want to know what it is.
>>>
>>> Since userspace already has an fd, expose the size using the
>>> size = lseek(fd, SEEK_END, 0); lseek(fd, SEEK_CUR, 0);
>>> idiom.
>>>
>>> v2: Added Daniel's sugeested documentation, with minor fixups
>>>
>>> Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@...onical.com>
>>> Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>
>>> Tested-by: Daniel Vetter <daniel.vetter@...ll.ch>
>>> ---
>>>   Documentation/dma-buf-sharing.txt | 12 ++++++++++++
>>>   drivers/base/dma-buf.c            | 28 ++++++++++++++++++++++++++++
>>>   2 files changed, 40 insertions(+)
>>>
>>> diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
>>> index 0b23261..849e982 100644
>>> --- a/Documentation/dma-buf-sharing.txt
>>> +++ b/Documentation/dma-buf-sharing.txt
>>> @@ -407,6 +407,18 @@ Being able to mmap an export dma-buf buffer object has 2 main use-cases:
>>>      interesting ways depending upong the exporter (if userspace starts depending
>>>      upon this implicit synchronization).
>>> +Other Interfaces Exposed to Userspace on the dma-buf FD
>>> +------------------------------------------------------
>>> +
>>> +- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
>>> +  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
>>> +  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
>>> +  llseek operation will report -EINVAL.
>>> +
>>> +  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
>> Shouldn't it be "supported" instead of "support"?
>>
>> Anyway, I'm just curious, in which case is it important to know the size?
>> Do we already have a way to get the dimensions (x, y and stripe)?
> Size is an invariant part of a dma-buf. Everything else wrt metadata isn't
> tracked in the kernel, so doesn't make much sense to expose it ;-)
> -Daniel
ACK, thanks Daniel.
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