Hi Tomasz,
On Monday 29 August 2011 10:01:58 Tomasz Stanislawski wrote:
> On 08/26/2011 05:09 PM, Laurent Pinchart wrote:
> > On Friday 26 August 2011 15:06:06 Tomasz Stanislawski wrote:
> >> This patch fix the handling of data passed to V4L2 ioctls. The content
> >> of the structures is not copied if the ioctl fails. It blocks ability
> >> to obtain any information about occurred error other then errno code.
> >> This patch fix this issue.
> >
> > Does the V4L2 spec say anything on this topic ? We might have
> > applications that rely on the ioctl argument structure not being touched
> > when a failure occurs.
>
> Ups.. I missed something. It looks that modifying ioctl content is
> illegal if ioctl fails. The spec says:
> "When an ioctl that takes an output or read/write parameter fails, the
> parameter remains unmodified." (v4l2 ioctl section)
> However, there is probably a bug already present in V4L2 framework.
> There are some ioctls that takes a pointer to an array as a field in the
> argument struct.
> The examples are all VIDIOC_*_EXT_CTRLS and VIDIOC_{QUERY/DQ/Q}_BUF family.
> The content of such an auxiliary arays is copied even if ioctl fails.
> Please take a look to video_usercopy function in v4l2-ioctl.c. Therefore
> I think that the spec is already violated. What is your opinion about
> this problem?
I think it was a bad idea to state that a parameter remains unmodified when
the ioctl fails in the first place. I'm fine with not following that for new
ioctls, but applications might rely on it for existing ioctls.
> Now back to selection case.
> This patch was added as proposition of fix to VIDIOC_S_SELECTION, to
> return the best-hit rectangle if constraints could not be satisfied. The
> ioctl return -ERANGE in this case. Using those return values the
> application gets some feedback on loosing constraints.
Shouldn't that always be the case ? :-) VIDIOC_S_SELECTION should adjust the
rectangle up or down depending on the constraints and always return the best
match without any error.
> I could remove rectangle returning from the spec and s5p-tv code for now.
--
Regards,
Laurent Pinchart
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