Re: [PATCH v8 01/29] drm/msm/dp: skip sink requests while the mainlink clocks are off

[Dmitry Baryshkov]

On Tue, Nov 10, 2026 at 12:54:22PM +0800, Yongxing Mou wrote:
> msm_dp_ctrl_handle_sink_request() programs REG_DP_STATE_CTRL from all
> three of its branches:
> 
>   DP_TEST_LINK_PHY_TEST_PATTERN -> msm_dp_ctrl_process_phy_test_request()
>                                    -> msm_dp_ctrl_off_link()
>   DP_LINK_STATUS_UPDATED        -> msm_dp_ctrl_link_maintenance()
>                                    -> msm_dp_ctrl_push_idle()
>   DP_TEST_LINK_TRAINING         -> msm_dp_ctrl_link_maintenance()
>                                    -> msm_dp_ctrl_push_idle()
> 
> That register lives in the mainlink clock domain, and nothing on the
> IRQ_HPD path guarantees those clocks are running. Disabling the
> boot-time mode for the first modeset from userspace turns the link
> clocks off. The sink answers the link going down with an IRQ_HPD
> carrying DP_LINK_STATUS_UPDATED (DPCD 0x204 = 0x80), and handling it
> walks straight into a write to an unclocked register block:
> 
>   [    4.572532] [drm:msm_dp_ctrl_off_link] disable
>   [    4.573756] [drm:msm_dp_ctrl_link_clk_disable] disabled link clocks
>   [    4.823194] [drm:msm_dp_link_process_request] sink request=0x80
>   [    4.823199] [drm:msm_dp_bridge_hpd_notify] 128
> 
> The board hung on 3 of 3 boots with a 5120x2160 SST sink attached over
> USB-C. On the hung boot captured with drm.debug=0x100 the printk ring
> ends at that last line, the drm_dbg_dp() immediately before
> msm_dp_ctrl_handle_sink_request(). In the RAM dumps CPU0 is still
> running pmic_glink_altmode_worker, holding the bridge chain mutex taken
> by drm_bridge_connector_hpd_notify(), and ctrl->link_clks_on is false.
> 
> Bail out when the mainlink clocks are off. All three branches need them
> running to do anything useful, so gating the function entry costs
> nothing: the first one resets the controller, which its own comment
> notes "will need DP link related clocks to be running", and the other
> two retrain the link.
> 
> Everything else reachable from the IRQ_HPD path - the MST ESI handling,
> msm_dp_display_process_hpd_high() and the video pattern test response -
> only talks to the sink over AUX and keeps working as before.
> 
> Reported-by: Joonhoe Kim <26rote@xxxxxxxxx>
> Closes: https://lore.kernel.org/all/20261108011501.4466-1-26rote@xxxxxxxxx/
> Fixes: c943b4948b58 ("drm/msm/dp: add displayPort driver support")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Yongxing Mou <yongxing.mou@xxxxxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/msm/dp/dp_ctrl.c | 6 ++++++
>  1 file changed, 6 insertions(+)

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxxxxxxxx>


-- 
With best wishes
Dmitry