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[PATCH v6 3/4] drm/dp_helper: Perform throw-away read before actual read in drm_dp_dpcd_read()

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This is part of a patch series to migrate all of the workarounds for
commonly seen behavior from bad sinks in intel_dp_dpcd_read_wake() to drm's
DP helper.

Some sinks will just return garbage for the first aux tranaction they
receive when coming out of sleep mode, so we need to perform an additional
read before the actual read to workaround this.

			    Changes since v5
- If the throwaway read in drm_dp_dpcd_read() fails, return the error
  from that instead of continuing. This follows the same logic we do in
  drm_dp_dpcd_access() (e.g. the error from the first transaction may
  differ from the errors that proceeding attempts might return).

Signed-off-by: Lyude <cpaul@redhat.com>
---
 drivers/gpu/drm/drm_dp_helper.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 540c3e4..eeaf5a7 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -248,6 +248,25 @@ unlock:
 ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
 			 void *buffer, size_t size)
 {
+	int ret;
+
+	/*
+	 * HP ZR24w corrupts the first DPCD access after entering power save
+	 * mode. Eg. on a read, the entire buffer will be filled with the same
+	 * byte. Do a throw away read to avoid corrupting anything we care
+	 * about. Afterwards things will work correctly until the monitor
+	 * gets woken up and subsequently re-enters power save mode.
+	 *
+	 * The user pressing any button on the monitor is enough to wake it
+	 * up, so there is no particularly good place to do the workaround.
+	 * We just have to do it before any DPCD access and hope that the
+	 * monitor doesn't power down exactly after the throw away read.
+	 */
+	ret = drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, DP_DPCD_REV, buffer,
+				 1);
+	if (ret != 1)
+		return ret;
+
 	return drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, offset, buffer,
 				  size);
 }
-- 
2.5.5

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Re: [PATCH v4 RESEND 0/5] Move workarounds from intel_dp_dpcd_read_wake() into drm's DP helpers Jani Nikula <jani.nikula@linux.intel.com> - 2016-04-12 12:20 +0200
  Re: [PATCH v4 RESEND 0/5] Move workarounds from  intel_dp_dpcd_read_wake() into drm's DP helpers Lyude <cpaul@redhat.com> - 2016-04-12 18:10 +0200
  [PATCH v5 0/4] Move workarounds from intel_dp_dpcd_read_wake() into drm's DP helpers Lyude <cpaul@redhat.com> - 2016-04-13 17:00 +0200
    [PATCH v5 4/4] drm/i915: Get rid of intel_dp_dpcd_read_wake() Lyude <cpaul@redhat.com> - 2016-04-13 17:00 +0200
      Re: [PATCH v5 4/4] drm/i915: Get rid of intel_dp_dpcd_read_wake() Daniel Vetter <daniel@ffwll.ch> - 2016-04-22 19:00 +0200
    [PATCH v5 1/4] drm/dp_helper: Always wait before retrying native aux transactions Lyude <cpaul@redhat.com> - 2016-04-13 17:00 +0200
    [PATCH v5 3/4] drm/dp_helper: Perform throw-away read before actual read in drm_dp_dpcd_read() Lyude <cpaul@redhat.com> - 2016-04-13 17:00 +0200
      Re: [PATCH v5 3/4] drm/dp_helper: Perform throw-away read before  actual read in drm_dp_dpcd_read() Ville Syrjälä <ville.syrjala@linux.intel.com> - 2016-04-14 19:10 +0200
        [PATCH v6 3/4] drm/dp_helper: Perform throw-away read before actual read in drm_dp_dpcd_read() Lyude <cpaul@redhat.com> - 2016-04-15 16:30 +0200
          Re: [PATCH v6 3/4] drm/dp_helper: Perform throw-away read before  actual read in drm_dp_dpcd_read() Ville Syrjälä <ville.syrjala@linux.intel.com> - 2016-04-15 16:40 +0200
    [PATCH v5 2/4] drm/dp_helper: Retry aux transactions on all errors Lyude <cpaul@redhat.com> - 2016-04-13 17:10 +0200

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