On 27/10/15 12:24, Russell King - ARM Linux wrote:
In passing, while reading the Intel DDX driver code, I've noticed that the Intel driver contains code which assumes that the master and render devices are related by minor device number, eg:/* Are we a render-node ourselves? */ if (is_render_node(fd, &master)) return NULL; sprintf(buf, "/dev/dri/renderD%d", (int)((master.st_rdev | 0x80) & 0xbf)); if (stat(buf, &render) == 0 && master.st_mode == render.st_mode && render.st_rdev == ((master.st_rdev | 0x80) & 0xbf)) return strdup(buf); There's also code doing the reverse as well. From my observations, the assumption that this code is built upon is false. I have an ARM platform here (non-Intel graphics) which shows the problem - we have a KMS-only DRM driver (card0) and a GPU-only DRM driver (card1). This populates the /dev/dri subdirectory as follows: crw-rw----+ 1 root video 226, 0 Oct 27 04:59 card0 crw-rw----+ 1 root video 226, 1 Oct 26 20:40 card1 crw-rw---- 1 root video 226, 64 Oct 26 20:40 controlD64 crw-rw---- 1 root video 226, 128 Oct 26 20:40 renderD128 and if I look at /sys/class/drm, you can then see who owns which devices: lrwxrwxrwx 1 root root 0 Oct 26 20:40 card0 -> ../../devices/platform/armada-drm/drm/card0 lrwxrwxrwx 1 root root 0 Oct 26 20:40 card0-HDMI-A-1 -> ../../devices/platform/armada-drm/drm/card0/card0-HDMI-A-1 lrwxrwxrwx 1 root root 0 Oct 26 20:40 card1 -> ../../devices/platform/etnaviv/drm/card1 lrwxrwxrwx 1 root root 0 Oct 26 20:40 controlD64 -> ../../devices/platform/armada-drm/drm/controlD64 lrwxrwxrwx 1 root root 0 Oct 26 20:40 renderD128 -> ../../devices/platform/etnaviv/drm/renderD128 So, renderD128 is card1's render node, which does not conform to the assumption which the Intel DDX driver makes - (1 | 0x80) & 0xbf is not 128. The same thing can happen if there's ever a case on Intel hardware where a KMS DRM driver registers prior to the i915 driver. I think the only way to properly determine which render nodes correspond with which master node is to actually open the device and check the device names, or parse sysfs - maybe reading the links of /sys/class/drm, and checking which link dirname corresponds with the master node. Any comments?
libdrm already has the right function for that:|char *drmGetRenderDeviceNameFromFd(int fd);
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