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

[Milan Zamazal]

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. */
+		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