Allow unprivileged use of radeontop via console

With appropriate privileges on the /dev/dri/cardN files, it is possible
to run radeontop as normal user. At minimum you need read/write
permissions:

    sudo setfacl -m u:$USER:rw /dev/dri/card*

You also need to be the DRM Master (i.e. Xorg must not be active on a
console).

README.md

@@ -3,7 +3,7 @@ RadeonTop
 
 View your GPU utilization, both for the total activity percent and individual blocks.
 
-Requires root rights or other permissions to /dev/mem.
+Requires access to /dev/dri/cardN files or /dev/mem (root privileges).
 
 Supported cards
 ---------------

auth.c

@@ -0,0 +1,31 @@
+/*
+    Copyright (C) 2016 Peter Wu <peter@lekensteyn.nl>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, version 3 of the License.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include "radeontop.h"
+
+void authenticate_drm(int fd) {
+	drm_magic_t magic;
+
+	/* Obtain magic for our DRM client. */
+	if (drmGetMagic(fd, &magic) < 0) {
+		return;
+	}
+
+	/* Try self-authenticate (if we are somehow the master). */
+	if (drmAuthMagic(fd, magic) == 0) {
+		return;
+	}
+}

detect.c

@@ -135,6 +135,7 @@ unsigned int init_pci(unsigned char bus, const unsigned char forcemem) {
 
 	use_ioctl = 0;
 	if (drm_fd >= 0) {
+		authenticate_drm(drm_fd);
 		uint32_t rreg = 0x8010;
 		use_ioctl = get_drm_value(drm_fd, RADEON_INFO_READ_REG, &rreg);
 	}

include/radeontop.h

@@ -48,6 +48,9 @@ enum {
 #define RADEON_INFO_READ_REG 0x24
 #endif
 
+// auth.c
+void authenticate_drm(int fd);
+
 // radeontop.c
 void die(const char *why);
 int get_drm_value(int fd, unsigned request, uint32_t *out);