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: Len Brown <lenb@kernel.org>
Cc: linux-pm@vger.kernel.org
---
 tools/power/x86/turbostat/turbostat.8 | 2 ++
 tools/power/x86/turbostat/turbostat.c | 4 ++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/tools/power/x86/turbostat/turbostat.8 b/tools/power/x86/turbostat/turbostat.8
index c7b26a3603af..3e1a4c4be001 100644
--- a/tools/power/x86/turbostat/turbostat.8
+++ b/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
diff --git a/tools/power/x86/turbostat/turbostat.c b/tools/power/x86/turbostat/turbostat.c
index aba460410dbd..fef76ed294fc 100644
--- a/tools/power/x86/turbostat/turbostat.c
+++ b/tools/power/x86/turbostat/turbostat.c
@@ -5476,13 +5476,13 @@ void print_dev_latency(void)
 
 	fd = open(path, O_RDONLY);
 	if (fd < 0) {
-		warn("fopen %s\n", path);
+		warn("open failed, try chmod +r %s\n", path);
 		return;
 	}
 
 	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;
 	}
-- 
2.38.1