[PATCH 3.13 143/198] drm/i915: Avoid div-by-zero when pixel_multiplier is zero

From: Kamal Mostafa
Date: Tue Jul 15 2014 - 17:44:39 EST -stable review patch. If anyone has any objections, please let me know.


From: =?UTF-8?q?Ville=20Syrj=C3=A4l=C3=A4?= <ville.syrjala@xxxxxxxxxxxxxxx>

commit 2b85886a5457f5c5dbcd32edbd4e6bba0f4e8678 upstream.

On certain platforms pixel_multiplier is read out in
.get_pipe_config(), but it also gets used to calculate the
pixel clock in intel_sdvo_get_config(). If the pipe is disable
but some SDVO outputs are active, we may end up dividing by zero
in intel_sdvo_get_config().

To avoid the problem simply check for zero pixel_multiplier and skip
the division. Another attempt at fixing this involved populating
pixel_multiplier to 1 even for disabled pipes, but that triggered a
WARN because SDVO_CMD_GET_CLOCK_RATE_MULT command failed and thus
encoder_pixel_multiplier was left at zero and didn't match

The "divide by pixel_multiplier" operation got introduced here:
commit 18442d08786472c63a0a80c27f92b033dffc26de
Author: Ville SyrjÃÂlÃÂ <ville.syrjala@xxxxxxxxxxxxxxx>
Date: Fri Sep 13 16:00:08 2013 +0300

drm/i915: Fix port_clock and adjusted_mode.clock readout all over

and it has caused a regression on certain machines since they would
hit the div-by-zero during resume.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76520
Tested-by: Tim Richardson <tim@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Signed-off-by: Ville SyrjÃlà <ville.syrjala@xxxxxxxxxxxxxxx>
Signed-off-by: Jani Nikula <jani.nikula@xxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
drivers/gpu/drm/i915/intel_sdvo.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_sdvo.c b/drivers/gpu/drm/i915/intel_sdvo.c
index a583e8f..9528239 100644
--- a/drivers/gpu/drm/i915/intel_sdvo.c
+++ b/drivers/gpu/drm/i915/intel_sdvo.c
@@ -1364,7 +1364,9 @@ static void intel_sdvo_get_config(struct intel_encoder *encoder,

- dotclock = pipe_config->port_clock / pipe_config->pixel_multiplier;
+ dotclock = pipe_config->port_clock;
+ if (pipe_config->pixel_multiplier)
+ dotclock /= pipe_config->pixel_multiplier;

if (HAS_PCH_SPLIT(dev))
ironlake_check_encoder_dotclock(pipe_config, dotclock);

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