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

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Message-ID: <20130904120504.GB28675@phenom.ffwll.local>
Date:	Wed, 4 Sep 2013 14:05:04 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Martin Peres <martin.peres@...e.fr>
Cc:	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)

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
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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