tools/power turbostat: Fix /dev/cpu_dma_latency warnings

When running as non-root the following error is seen in turbostat:

turbostat: fopen /dev/cpu_dma_latency
: Permission denied

turbostat and the man page have information on how to avoid other
permission errors, so these can be fixed the same way.

Provide better /dev/cpu_dma_latency warnings that provide instructions on
how to avoid the error, and update the man page.
Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: linux-pm@vger.kernel.org
Signed-off-by: Len Brown <len.brown@intel.com>

tools/power/x86/turbostat/turbostat.8

@@ -344,6 +344,8 @@ Alternatively, non-root users can be enabled to run turbostat this way:
 
 # chmod +r /dev/cpu/*/msr
 
+# chmod +r /dev/cpu_dma_latency
+
 .B "turbostat "
 reads hardware counters, but doesn't write them.
 So it will not interfere with the OS or other programs, including

tools/power/x86/turbostat/turbostat.c

@@ -5482,7 +5482,7 @@ void print_dev_latency(void)
 
 	retval = read(fd, (void *)&value, sizeof(int));
 	if (retval != sizeof(int)) {
-		warn("read %s\n", path);
+		warn("read failed %s\n", path);
 		close(fd);
 		return;
 	}