[PATCH v2] apps: cam: Skip devices without connectors

[Milan Zamazal]

Hi Laurent,

Laurent Pinchart <laurent.pinchart at ideasonboard.com> writes:

> Hi Milan,
>
> Thank you for the patch.
>
> On Thu, May 15, 2025 at 06:25:36PM +0200, Milan Zamazal wrote:
>> Device::openCard() in the cam DRM helpers looks for a /dev/dri/card*
>> device that can be opened and that doesn't fail when asked about
>> DRM_CAP_DUMB_BUFFER capability (regardless whether the capability is
>> supported by the device).
>> 
>> There can be matching devices that are not display devices.  This can
>
> Can you share an example of such a device ? Are we talking about a
> device that sets DRIVER_MODESET but is not a display device ?

I guess so.  I have the following on TI AM69:

  # ls -l /dev/dri/
  total 0
  drwxr-xr-x 2 root root        100 Mar  7 01:00 by-path/
  crw-rw---- 1 root video  226,   0 Mar  7 01:00 card0
  crw-rw---- 1 root video  226,   1 Mar  7 01:00 card1
  crw-rw-rw- 1 root render 226, 128 Mar  7 01:00 renderD128

  # ls -l /sys/class/drm/
  total 0
  lrwxrwxrwx 1 root root    0 Mar  7 01:00 card0 -> '../../devices/platform/bus at 100000/4a00000.dss/drm/card0'/
  lrwxrwxrwx 1 root root    0 Mar  7 01:00 card0-DP-1 -> '../../devices/platform/bus at 100000/4a00000.dss/drm/card0/card0-DP-1'/
  lrwxrwxrwx 1 root root    0 Mar  7 01:00 card0-HDMI-A-1 -> '../../devices/platform/bus at 100000/4a00000.dss/drm/card0/card0-HDMI-A-1'/
  lrwxrwxrwx 1 root root    0 May 16 14:31 card1 -> '../../devices/platform/bus at 100000/4e20000000.gpu/drm/card1'/
  lrwxrwxrwx 1 root root    0 May 16 14:31 renderD128 -> '../../devices/platform/bus at 100000/4e20000000.gpu/drm/renderD128'/
  -r--r--r-- 1 root root 4096 May 16 16:49 version

card1 causes the problem.

>> lead to selection of such a device and inability to use KMS output with
>> `cam' application.  The ultimate goal is to display something on the
>> device and later the KMS sink will fail if there is no connector
>> attached to the device (although it can actually fail earlier, when
>> trying to set DRM_CLIENT_CAP_ATOMIC capability if this is not
>> supported).  Let's avoid selecting devices without connectors.
>> 
>> A question is whether the added check makes the check for
>> DRM_CAP_DUMB_BUFFER API redundant or not.
>> 
>> Changes in v2:
>> - Rebased on current master.
>> 
>> Signed-off-by: Milan Zamazal <mzamazal at redhat.com>
>> ---
>>  src/apps/cam/drm.cpp | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>> 
>> diff --git a/src/apps/cam/drm.cpp b/src/apps/cam/drm.cpp
>> index 47bbb6b0..e19e848c 100644
>> --- a/src/apps/cam/drm.cpp
>> +++ b/src/apps/cam/drm.cpp
>> @@ -479,6 +479,18 @@ int Device::openCard()
>>  			continue;
>>  		}
>>  
>> +		/* Skip devices without connectors. */
>> +		std::unique_ptr<drmModeRes, decltype(&drmModeFreeResources)> resources{
>> +			drmModeGetResources(fd_),
>> +			&drmModeFreeResources
>> +		};
>> +		if (!resources || resources->count_connectors <= 0) {
>> +			resources.reset();
>> +			drmClose(fd_);
>> +			fd_ = -1;
>> +			continue;
>> +		}
>> +
>>  		found = true;
>>  		break;
>>  	}