[PATCH] drm/msm/dsi: don't re-lock the PHY PLL on every DSI command

[Kavan Smith <kavansmith82@gmail.com>]

msm_dsi_host_xfer_prepare() runs for every DSI command, including runtime
DCS writes such as backlight (MIPI DCS 0x51), and unconditionally calls
link_clk_set_rate() before enabling the link clocks. On MSM8916 (DSI 6G
v1.3.1) the requested byte-clock rate never exactly equals the DSI PHY
PLL's achievable rate (e.g. 56250000 Hz requested vs 56246337 Hz from the
PLL), so the clk framework treats every call as a rate change and re-locks
the PLL.

During video-mode operation the byte, pixel and byte-intf clocks are
already running and feeding continuous scanout. Re-locking the PLL glitches
that live clock. On a video-mode panel with no internal timing generator
(e.g. samsung,s6d7aa0 / lsl080al03 on the Samsung Galaxy Tab A 8.0, which
regenerates its H/V timing directly from the DSI clock lane) the glitch
makes the panel lose pixel lock, producing ~1 second of displaced/wrapped
scanout on every DCS command. No FIFO underrun or dsi_err_worker error
fires; it is a silent clock-domain glitch. Stock (downstream MDSS) firmware
sends the same 0x51 with CMD_CLK_CTRL, which only refcount-enables the
clocks and never re-runs clk_set_rate, and does not glitch.

The link rate is already programmed at power-on by msm_dsi_host_power_on(),
so skip the redundant re-set once the link is up; init-time transfers
(before power_on) still set the rate.

Link: https://lists.freedesktop.org/archives/dri-devel/2018-April/172218.html
Signed-off-by: Kavan Smith <kavansmith82@gmail.com>
---
 drivers/gpu/drm/msm/dsi/dsi_host.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
--- a/drivers/gpu/drm/msm/dsi/dsi_host.c
+++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
@@ -2170,7 +2170,17 @@
 	 * mdp clock need to be enabled to receive dsi interrupt
 	 */
 	pm_runtime_get_sync(&msm_host->pdev->dev);
-	cfg_hnd->ops->link_clk_set_rate(msm_host);
+	/*
+	 * Don't re-set the link clock rate when the link is already up. The
+	 * requested byte-clock rate rarely equals the DSI PHY PLL's achievable
+	 * rate, so clk_set_rate() re-locks the PLL on every command; for a
+	 * video-mode panel with no internal timing generator that clock glitch
+	 * makes the panel lose pixel lock mid-scanout (~1s of displaced image on
+	 * each DCS write, e.g. every backlight update). The rate is already set
+	 * at power-on.
+	 */
+	if (!msm_host->power_on)
+		cfg_hnd->ops->link_clk_set_rate(msm_host);
 	cfg_hnd->ops->link_clk_enable(msm_host);
 
 	/* TODO: vote for bus bandwidth */