Em Wed, 26 Dec 2012 12:33:58 +0100
Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> escreveu:
> Hi Hans,
>
> On Tuesday 25 December 2012 12:50:51 Hans Verkuil wrote:
> > On Tue December 25 2012 12:23:00 Laurent Pinchart wrote:
> > > On Tuesday 25 December 2012 12:15:25 Hans Verkuil wrote:
> > > > On Mon December 24 2012 13:27:08 Laurent Pinchart wrote:
> > > > > On Thursday 27 September 2012 17:16:15 Laurent Pinchart wrote:
> > > > > > When one of the requested controls doesn't exist the error_idx field
> > > > > > must reflect that situation. For G_EXT_CTRLS and S_EXT_CTRLS,
> > > > > > error_idx must be set to the control count. For TRY_EXT_CTRLS, it
> > > > > > must be set to the index of the unexisting control.
> > > > > >
> > > > > > This issue was found by the v4l2-compliance tool.
> > > > >
> > > > > I'm revisiting this patch as it has been reverted in v3.8-rc1.
> > > > >
> > > > > > Signed-off-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> > > > > > ---
> > > > > >
> > > > > > drivers/media/usb/uvc/uvc_ctrl.c | 17 ++++++++++-------
> > > > > > drivers/media/usb/uvc/uvc_v4l2.c | 19 ++++++++++++-------
> > > > > > 2 files changed, 22 insertions(+), 14 deletions(-)
> > > > >
> > > > > [snip]
> > > > >
> > > > > > diff --git a/drivers/media/usb/uvc/uvc_v4l2.c
> > > > > > b/drivers/media/usb/uvc/uvc_v4l2.c index f00db30..e5817b9 100644
> > > > > > --- a/drivers/media/usb/uvc/uvc_v4l2.c
> > > > > > +++ b/drivers/media/usb/uvc/uvc_v4l2.c
> > > > > > @@ -591,8 +591,10 @@ static long uvc_v4l2_do_ioctl(struct file
> > > > > > *file,
> > > > >
> > > > > [snip]
> > > > >
> > > > > > @@ -637,8 +639,9 @@ static long uvc_v4l2_do_ioctl(struct file *file,
> > > > > > unsigned int cmd, void *arg) ret = uvc_ctrl_get(chain, ctrl);
> > > > > > if (ret < 0) {
> > > > > > uvc_ctrl_rollback(handle);
> > > > > >
> > > > > > - ctrls->error_idx = i;
> > > > > > - return ret;
> > > > > > + ctrls->error_idx = ret == -ENOENT
> > > > > > + ? ctrls->count : i;
> > > > > > + return ret == -ENOENT ? -EINVAL : ret;
> > > > > > }
> > > > > > }
> > > > > > ctrls->error_idx = 0;
> > > > > > @@ -661,8 +664,10 @@ static long uvc_v4l2_do_ioctl(struct file
> > > > > > *file,
> > > > > > unsigned int cmd, void *arg) ret = uvc_ctrl_set(chain, ctrl);
> > > > > > if (ret < 0) {
> > > > > > uvc_ctrl_rollback(handle);
> > > > > >
> > > > > > - ctrls->error_idx = i;
> > > > > > - return ret;
> > > > > > + ctrls->error_idx = (ret == -ENOENT &&
> > > > > > + cmd == VIDIOC_S_EXT_CTRLS)
> > > > > > + ? ctrls->count : i;
> > > > > > + return ret == -ENOENT ? -EINVAL : ret;
> > > > > > }
> > > > > > }
> > > > >
> > > > > I've reread the V4L2 specification, and the least I can say is that
> > > > > the text is pretty ambiguous. Let's clarify it.
> > > > >
> > > > > Is there a reason to differentiate between invalid control IDs and
> > > > > other errors as far as error_idx is concerned ? It would be simpler if
> > > > > error_idx was set to the index of the first error for get and try
> > > > > operations, regardless of the error type. What do you think ?
> > > >
> > > > There is a good reason for doing this: the G/S_EXT_CTRLS ioctls have to
> > > > be as atomic as possible, i.e. it should try hard to prevent leaving the
> > > > hardware in an inconsistent state because not all controls could be set.
> > > > It can never be fully atomic since writing multiple registers over usb
> > > > or i2c can always return errors for one of those writes, but it should
> > > > certainly check for all the obvious errors first that do not require
> > > > actually writing to the hardware, such as whether all the controls in
> > > > the control list actually exist.
> > > >
> > > > And for such errors error_idx should be set to the number of controls to
> > > > indicate that none of the controls were actually set but that there was
> > > > a problem with the list of controls itself.
> > >
> > > For S_EXT_CTRLS, sure, but G_EXT_CTRLS doesn't modify the hardware state,
> > > so it could get all controls up to the erroneous one.
> >
> > I have thought about that but I decided against it. One reason is to have
> > get and set behave the same since both access the hardware. The other
> > reason is that even getting a control value might change the hardware
> > state, for example by resetting some internal hardware counter when a
> > register is read (it's rare but there is hardware like that). Furthermore,
> > reading hardware registers can be slow so why not do the sanity check
> > first?
>
> Get can indeed change the device state in rare cases, but the information
> won't be lost, as the value of all controls before error_idx will be returned.
Huh? reading a control should never alter the device's state. If the hardware
is resetting a register, then such register should be shadowed, and some other
way to explicitly reset its value should be used.
> What bothers me with the current G_EXT_CTRLS implementation (beside that it's
> very slightly more complex for the uvcvideo driver than the one I propose) is
> that an application will have no way to know for which control G_EXT_CTRLS
> failed. This is especially annoying during development.
>
> Maybe we could leave this behaviour as driver-specific ?
driver-specific behavior for IOCTL's should be avoided, as applications will
fail if they see something it doesn't expect.
--
Cheers,
Mauro
--
To unsubscribe from this list: send the line "unsubscribe linux-media" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
