Hi Will, On Thursday 13 March 2014 10:48:20 Will Manley wrote: > On Thu, 13 Mar 2014, at 10:23, Laurent Pinchart wrote: > > First of all, could you please CC me in the future for uvcvideo patches ? > > Will do. Thank you. > > On Wednesday 12 March 2014 18:08:31 William Manley wrote: > > > The uvcvideo webcam driver exposes the v4l2 control "Exposure > > > (Absolute)" which allows the user to control the exposure time of the > > > webcam, essentially controlling the brightness of the received image. > > > By default the webcam automatically adjusts the exposure time > > > automatically but the if you set the control "Exposure, Auto"="Manual > > > Mode" the user can fix the exposure time. > > > > > > Unfortunately it seems that the Logitech C920 has a firmware bug where > > > it will forget that it's in manual mode temporarily during > > > initialisation. This means that the camera doesn't respect the exposure > > > time that the user requested if they request it before starting to > > > stream video. They end up with a video stream which is either too bright > > > or too dark and must reset the controls after video starts streaming. > > > > I've asked Logitech whether they can confirm this is a known issue. I'm > > not sure when I'll have a reply though. > > Great :) > > > > This patch works around this camera bug by re-uploading the cached > > > controls to the camera immediately after initialising the camera. > > > > I'm a bit concerned about this. As you noticed UVC camera are often buggy, > > and small changes in the driver can fix problems with one model and break > > others. Sending a bunch of SET_CUR requests at once right after starting > > the stream is something that has the potential to crash firmwares (yes, > > they can be that fragile, unfortunately). > > Good point. I can add a quirk such that it only happens with the C920. > > > I would like to get a better understanding of the problem first. As I > > don't have a C920, could you please perform two tests for me ? > > > > I would first like to know what the camera reports as its exposure time > > after starting the stream. If you get the exposure time at that point (by > > sending a GET_CUR request, bypassing the driver cache), do you get the > > value you had previously set (which, from your explanation, would be > > incorrect, as the exposure time has changed based on your findings), or a > > different value ? Does the camera change the exposure priority control > > autonomously as well, or just the exposure time ? > > It's a bit of a strange behaviour. I'd already tried littering the code with > (uncached) GET_CUR requests. It seems that the value changes sometime during > the call to usb_set_interface in uvc_init_video. I'll assume this means that the camera reports the updated exposure time in response to the GET_CUR request. Does the value of other controls (such as the exposure priority control for instance) change as well ? > Strangely enough though calling uvc_ctrl_restore_values immediately after > uvc_init_video has no effect. It must be put after the usb_submit_urb loop > to fix the problem. > > > Then, I would like to know whether the camera sends a control update > > event when you start the stream, or if it just changes the exposure time > > without notifying the driver. > > Wireshark tells me that it is sending a control update event, but it seems > like uvcvideo ignores it. I had to add the flag UVC_CTRL_FLAG_AUTO_UPDATE to > the uvc_control_info entry for "Exposure (Auto)" for the new value to be > properly reported to userspace. Could you send me the USB trace ? -- Regards, Laurent Pinchart -- 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
