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

[Dmitry Baryshkov]

On Mon, Jul 06, 2026 at 11:07:53AM -0700, Kavan Smith wrote:
> 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.

THe problem is that by definition the transfer callbacks can be sent at
any time, see the comment at mipi_dsi_host_ops. I think the proper fix
would be to call clk_round_rate() beforehand, making the CCF skip all
further rate sets.


> 
> Link: https://lists.freedesktop.org/archives/dri-devel/2018-April/172218.html

This should be Reported-by / Closes. Also probably add Fixes and cc:stable.

> Signed-off-by: Kavan Smith <kavansmith82@xxxxxxxxx>
> ---
>  drivers/gpu/drm/msm/dsi/dsi_host.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 

-- 
With best wishes
Dmitry