From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from perceval.ideasonboard.com ([95.142.166.194]:56855 "EHLO perceval.ideasonboard.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751347Ab2LZLch (ORCPT ); Wed, 26 Dec 2012 06:32:37 -0500 From: Laurent Pinchart To: Hans Verkuil Cc: linux-media@vger.kernel.org, Hans Verkuil Subject: Re: [PATCH 1/6] uvcvideo: Set error_idx properly for extended controls API failures Date: Wed, 26 Dec 2012 12:33:58 +0100 Message-ID: <22611337.csYnEZHssR@avalon> In-Reply-To: <201212251250.51838.hverkuil@xs4all.nl> References: <1348758980-21683-1-git-send-email-laurent.pinchart@ideasonboard.com> <1427386.yhbGQRN2rP@avalon> <201212251250.51838.hverkuil@xs4all.nl> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-media-owner@vger.kernel.org List-ID: 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 > > > > > --- > > > > > > > > > > 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. 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 ? -- Regards, Laurent Pinchart