[PATCH v3] apps: cam: Skip non-display GPUs

[Kieran Bingham]

Quoting Milan Zamazal (2025-05-29 09:23:26)
> 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
> 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, CRTCs or encoders.
> The added check makes the original check for DRM_CAP_DUMB_BUFFER API
> most likely unnecessary, let's remove it.
> 
> Changes in v3:
> - Added checks for the presence of CRTCs and encoders.
> - Removed the check for DRM_CAP_DUMB_BUFFER.
> 
> Signed-off-by: Milan Zamazal <mzamazal at redhat.com>
> ---
>  src/apps/cam/drm.cpp | 22 ++++++++++------------
>  1 file changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/src/apps/cam/drm.cpp b/src/apps/cam/drm.cpp
> index 47bbb6b05..c40c31410 100644
> --- a/src/apps/cam/drm.cpp
> +++ b/src/apps/cam/drm.cpp
> @@ -450,8 +450,6 @@ int Device::openCard()
>         }
>  
>         for (struct dirent *res; (res = readdir(folder));) {
> -               uint64_t cap;
> -
>                 if (strncmp(res->d_name, "card", 4))
>                         continue;
>  
> @@ -464,16 +462,16 @@ int Device::openCard()
>                         continue;
>                 }
>  
> -               /*
> -                * Skip devices that don't support the modeset API, to avoid
> -                * selecting a DRM device corresponding to a GPU. There is no
> -                * modeset capability, but the kernel returns an error for most
> -                * caps if mode setting isn't support by the driver. The
> -                * DRM_CAP_DUMB_BUFFER capability is one of those, other would
> -                * do as well. The capability value itself isn't relevant.
> -                */
> -               ret = drmGetCap(fd_, DRM_CAP_DUMB_BUFFER, &cap);
> -               if (ret < 0) {
> +               /* Skip devices without connectors. */

This all sounds reasonable to me, though this comment now skips devices
without connectors/crts/encoders.

Perhaps we should update it while applying?

		/* Skip non-display devices */


Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

> +               std::unique_ptr<drmModeRes, decltype(&drmModeFreeResources)> resources{
> +                       drmModeGetResources(fd_),
> +                       &drmModeFreeResources
> +               };
> +               if (!resources ||
> +                   resources->count_connectors <= 0 ||
> +                   resources->count_crtcs <= 0 ||
> +                   resources->count_encoders <= 0) {
> +                       resources.reset();
>                         drmClose(fd_);
>                         fd_ = -1;
>                         continue;
> -- 
> 2.49.0
>