[PATCH 2/2] Revert "drm/i915/dp: change aux_ctl reg read to polling read"

[Ville Syrjala <ville.syrjala@linux.intel.com>]

From: Ville Syrjälä <ville.syrjala@linux.intel.com>

This reverts commit 5a9b0c7418448ed3766f61ba0a71d08f259c3181.

The switch from AUX interrupts to pollign was very hand-wavy.
Yes, there have been some situations in CI on a few platforms
where the AUX hardware seemingly forgets to signal the timeout,
but those have been happening after we switched to polling as
well. So I don't think we have any conclusive evidence that
polling actually helps here.

Someone really should root cause the actual problem, and see
if there is a proper workaround we could implemnt (eg. disabling
clock gating/etc.). In the meantime just go back to using the
interrupt for AUX completion.

If the hardware fails to signal the timeout we will just hit
the wait_event_timeout() software timeout instead. I suppose
we could try to tune the software timeout to more closely
match the expected hardware timeout. Might need to use
wait_event_hrtimeout() or something to avoid jiffies
granularity issues...

The AUX polling is also a hinderance towards using poll_timeout_us()
because we have a very long timeout, but would need a fairly short
polling interval to keep AUX transfer reasonably fast. Someone would
need to come up with good numbers in a somewhat scientific way.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
---
 drivers/gpu/drm/i915/display/intel_dp_aux.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux.c b/drivers/gpu/drm/i915/display/intel_dp_aux.c
index 809799f63e32..d1a93e4a59b5 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux.c
@@ -6,6 +6,7 @@
 #include <drm/drm_print.h>
 
 #include "intel_de.h"
+#include "intel_display_jiffies.h"
 #include "intel_display_types.h"
 #include "intel_display_utils.h"
 #include "intel_dp.h"
@@ -60,16 +61,17 @@ intel_dp_aux_wait_done(struct intel_dp *intel_dp)
 	i915_reg_t ch_ctl = intel_dp->aux_ch_ctl_reg(intel_dp);
 	const unsigned int timeout_ms = 10;
 	u32 status;
-	int ret;
+	bool done;
 
-	ret = intel_de_wait_ms(display, ch_ctl,
-			       DP_AUX_CH_CTL_SEND_BUSY, 0,
-			       timeout_ms, &status);
+#define C (((status = intel_de_read_notrace(display, ch_ctl)) & DP_AUX_CH_CTL_SEND_BUSY) == 0)
+	done = wait_event_timeout(display->gmbus.wait_queue, C,
+				  msecs_to_jiffies_timeout(timeout_ms));
 
-	if (ret == -ETIMEDOUT)
+	if (!done)
 		drm_err(display->drm,
 			"%s: did not complete or timeout within %ums (status 0x%08x)\n",
 			intel_dp->aux.name, timeout_ms, status);
+#undef C
 
 	return status;
 }
-- 
2.49.1