[PATCH v4] apps: cam: Skip non-display GPUs

[Laurent Pinchart]

On Fri, May 30, 2025 at 11:08:15AM +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
> 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.
> 
> Signed-off-by: Milan Zamazal <mzamazal at redhat.com>
> Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> Reviewed-by: Mattijs Korpershoek <mkorpershoek at kernel.org>
> Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

Pushed, thank you.

> ---
>  src/apps/cam/drm.cpp | 25 +++++++++++++++----------
>  1 file changed, 15 insertions(+), 10 deletions(-)
> 
> diff --git a/src/apps/cam/drm.cpp b/src/apps/cam/drm.cpp
> index 47bbb6b05..f4b470976 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;
>  
> @@ -465,15 +463,22 @@ int Device::openCard()
>  		}
>  
>  		/*
> -		 * 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.
> +		 * Skip non-display devices. While this could in theory be done
> +		 * by checking for support of the mode setting API, some
> +		 * out-of-tree render-only GPU drivers (namely powervr)
> +		 * incorrectly set the DRIVER_MODESET driver feature. Check for
> +		 * the presence of at least one CRTC, encoder and connector
> +		 * instead.
>  		 */
> -		ret = drmGetCap(fd_, DRM_CAP_DUMB_BUFFER, &cap);
> -		if (ret < 0) {
> +		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;

-- 
Regards,

Laurent Pinchart