Re: [PATCH 1/6] uvcvideo: Set error_idx properly for extended controls API failures

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Hans,

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@ideasonboard.com>
> > > ---
> > > 
> > >  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.

> Since TRY_EXT_CTRLS doesn't touch the hardware at all the error_idx can be
> set to the index of the control that caused the problem.
> 
> The documentation can definitely be improved as the difference between G/S
> and TRY with regards to error_idx is not made explicit.

-- 
Regards,

Laurent Pinchart