[PATCH] drm/i915/vlv: Add cdclk workaround for DSI

[Hans de Goede]

At least on the Chuwi Vi8 (non pro/plus) the LCD panel will show an image
shifted aprox. 20% to the left (with wraparound) and sometimes also wrong
colors, showing that the panel controller is starting with sampling the
datastream somewhere mid-line. This happens after the first blanking and
re-init of the panel.

After looking at drm.debug output I noticed that initially we inherit the
cdclk of 333333 KHz set by the GOP, but after the re-init we picked 266667
KHz, which turns out to be the cause of this problem, a quick hack to hard
code the cdclk to 333333 KHz makes the problem go away.

I've tested this on various Bay Trail devices:
-Chuwi Vi8: 800x1280, 333333 at boot, requires 333333 to work properly
-GP-electronic T701: 1024x600, 333333 at boot, works fine with 333333
-PEAQ C1010: 1920x1200, 333333 at boot, works fine with 333333
-PoV mobii-wintab-800w: 800x1280, 333333 at boot, works fine with 333333
-Asus Transformer-T100TA: 1368x768, 320000 at boot, works fine with 320000

So it seems that the GOP is always using what vlv_calc_cdclk calls
freq_320 as cdclk clock. Also note the comment in vlv_calc_cdclk about
it avoiding 200 Mhz as clock because that causes issues in some cases.

This commit extends the "do not use 200 Mhz" workaround with an extra
check to require atleast 320000 KHz (avoiding 266667 KHz) when a DSI
panel is active.

Signed-off-by: Hans de Goede <hdegoede at redhat.com>
---
 drivers/gpu/drm/i915/intel_cdclk.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_cdclk.c b/drivers/gpu/drm/i915/intel_cdclk.c
index 9c5ceb98d48f..cfb3f7fb3e1c 100644
--- a/drivers/gpu/drm/i915/intel_cdclk.c
+++ b/drivers/gpu/drm/i915/intel_cdclk.c
@@ -1923,6 +1923,14 @@ int intel_crtc_compute_min_cdclk(const struct intel_crtc_state *crtc_state)
 	if (crtc_state->has_audio && INTEL_GEN(dev_priv) >= 9)
 		min_cdclk = max(2 * 96000, min_cdclk);
 
+	/*
+	 * On Valleyview the GOP always uses 320000/333333 KHz if DSI is used,
+	 * using a lower clock causes causes sync issues with some panels.
+	 */
+	if (intel_crtc_has_type(crtc_state, INTEL_OUTPUT_DSI) &&
+	    IS_VALLEYVIEW(dev_priv))
+		min_cdclk = max(320000, min_cdclk);
+
 	if (min_cdclk > dev_priv->max_cdclk_freq) {
 		DRM_DEBUG_KMS("required cdclk (%d kHz) exceeds max (%d kHz)\n",
 			      min_cdclk, dev_priv->max_cdclk_freq);
-- 
2.14.3