[libcamera-devel] [PATCH] cam: drm: Skip DRM devices not capable of supporting the atomic API

[Laurent Pinchart]

The DRM helper picks the first DRM card that it can open. On platforms
that have a standalone GPU, this risks selecting a device corresponding
to the GPU instead of the display controller. Fix this by skipping
devices that don't support the KMS atomic API. Some legacy display
controllers would be skipped as well, but libcamera doesn't run on those
systems anyway, and the DRM helper doesn't support the legacy KMS
modeset API in any case.

Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
 src/cam/drm.cpp | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/src/cam/drm.cpp b/src/cam/drm.cpp
index b0602c942853..b2a6b0bba9e8 100644
--- a/src/cam/drm.cpp
+++ b/src/cam/drm.cpp
@@ -430,7 +430,8 @@ int Device::init()
 int Device::openCard()
 {
 	const std::string dirName = "/dev/dri/";
-	int ret = -ENOENT;
+	bool found = false;
+	int ret;
 
 	/*
 	 * Open the first DRM/KMS device beginning with /dev/dri/card. The
@@ -449,24 +450,38 @@ int Device::openCard()
 	}
 
 	for (struct dirent *res; (res = readdir(folder));) {
+		uint64_t cap;
+
 		if (strncmp(res->d_name, "card", 4))
 			continue;
 
 		const std::string devName = dirName + res->d_name;
 		fd_ = open(devName.c_str(), O_RDWR | O_CLOEXEC);
-		if (fd_ >= 0) {
-			ret = 0;
-			break;
+		if (fd_ < 0) {
+			ret = -errno;
+			std::cerr << "Failed to open DRM/KMS device " << devName << ": "
+				  << strerror(-ret) << std::endl;
+			continue;
 		}
 
-		ret = -errno;
-		std::cerr << "Failed to open DRM/KMS device " << devName << ": "
-			  << strerror(-ret) << std::endl;
+		/*
+		 * Skip devices that don't support the atomic API, to avoid
+		 * selecting a DRM device corresponding to a GPU.
+		 */
+		ret = drmGetCap(fd_, DRM_CLIENT_CAP_ATOMIC, &cap);
+		if (ret < 0) {
+			drmClose(fd_);
+			fd_ = -1;
+			continue;
+		}
+
+		found = true;
+		break;
 	}
 
 	closedir(folder);
 
-	return ret;
+	return found ? 0 : -ENOENT;
 }
 
 int Device::getResources()
-- 
Regards,

Laurent Pinchart