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

[Martin Peres]

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@xxxxxxxxxxxxx>
> > > Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > > Tested-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > > ---
> > >   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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