Re: [PATCH v3 17/38] drm/msm/dp: add support to send ACT packets for MST

[Yongxing Mou <yongxing.mou@oss.qualcomm.com>]

On 4/1/2026 7:27 PM, Dmitry Baryshkov wrote:
> On Wed, Apr 01, 2026 at 02:55:32PM +0800, Yongxing Mou wrote:
>>
>>
>> On 4/1/2026 2:47 PM, Dmitry Baryshkov wrote:
>>> On 01/04/2026 09:44, Yongxing Mou wrote:
>>>>
>>>>
>>>> On 8/26/2025 5:10 AM, Dmitry Baryshkov wrote:
>>>>> On Mon, Aug 25, 2025 at 10:16:03PM +0800, Yongxing Mou wrote:
>>>>>> From: Abhinav Kumar <quic_abhinavk@quicinc.com>
>>>>>>
>>>>>> Whenever virtual channel slot allocation changes, the DP
>>>>>> source must send the action control trigger sequence to notify
>>>>>> the sink about the same. This would be applicable during the
>>>>>> start and stop of the pixel stream. Add the infrastructure
>>>>>> to be able to send ACT packets for the DP controller when
>>>>>> operating in MST mode.
>>>>>>
>>>>>> Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
>>>>>> Signed-off-by: Yongxing Mou <yongxing.mou@oss.qualcomm.com>
>>>>>> ---
>>>>>>    drivers/gpu/drm/msm/dp/dp_ctrl.c    | 39
>>>>>> +++++++++++++++++++++++++ + +++++++++--
>>>>>>    drivers/gpu/drm/msm/dp/dp_ctrl.h    |  4 ++--
>>>>>>    drivers/gpu/drm/msm/dp/dp_display.c |  3 ++-
>>>>>>    drivers/gpu/drm/msm/dp/dp_display.h |  1 +
>>>>>>    drivers/gpu/drm/msm/dp/dp_reg.h     |  2 ++
>>>>>>    5 files changed, 44 insertions(+), 5 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c
>>>>>> b/drivers/gpu/drm/msm/ dp/dp_ctrl.c
>>>>>> index 608a1a077301b2ef3c77c271d873bb4364abe779..16e5ed58e791971d5dca3077cbb77bfcc186505a
>>>>>> 100644
>>>>>> --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
>>>>>> +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c
>>>>>> @@ -142,6 +142,7 @@ struct msm_dp_ctrl_private {
>>>>>>        bool core_clks_on;
>>>>>>        bool link_clks_on;
>>>>>>        bool stream_clks_on[DP_STREAM_MAX];
>>>>>> +    bool mst_active;
>>>>>>    };
>>>>>>    static inline u32 msm_dp_read_ahb(const struct
>>>>>> msm_dp_ctrl_private *ctrl, u32 offset)
>>>>>> @@ -227,6 +228,32 @@ static int
>>>>>> msm_dp_aux_link_configure(struct drm_dp_aux *aux,
>>>>>>        return 0;
>>>>>>    }
>>>>>> +void msm_dp_ctrl_mst_send_act(struct msm_dp_ctrl *msm_dp_ctrl)
>>>>>> +{
>>>>>> +    struct msm_dp_ctrl_private *ctrl;
>>>>>> +    bool act_complete;
>>>>>> +
>>>>>> +    ctrl = container_of(msm_dp_ctrl, struct
>>>>>> msm_dp_ctrl_private, msm_dp_ctrl);
>>>>>> +
>>>>>> +    if (!ctrl->mst_active)
>>>>>> +        return;
>>>>>> +
>>>>>> +    msm_dp_write_link(ctrl, REG_DP_MST_ACT, 0x1);
>>>>>> +    /* make sure ACT signal is performed */
>>>>>> +    wmb();
>>>>>> +
>>>>>> +    msleep(20); /* needs 1 frame time */
>>>>>> +
>>>>>> +    act_complete = msm_dp_read_link(ctrl, REG_DP_MST_ACT);
>>>>>> +
>>>>>> +    if (!act_complete)
>>>>>> +        drm_dbg_dp(ctrl->drm_dev, "mst ACT trigger complete
>>>>>> SUCCESS\n");
>>>>>> +    else
>>>>>> +        drm_dbg_dp(ctrl->drm_dev, "mst ACT trigger complete
>>>>>> failed\n");
>>>>>
>>>>> Shouldn't it return an error if the register dind't latch? Also,
>>>>> shouldn't we set mst_active only if the write went through?
>>>>>
>>>> In some cases, MST still works correctly even when the ACT trigger
>>>> fails; here refer to the downstream implementation.
>>>
>>> I don't think it is a good idea. It would be better to signal this to
>>> the user and rollback the MST configuration (as in the case of any other
>>> error).
>>>
>>> I will change my mind if you point out i915, amdgpu or nouveau drivers
>>> ignoring the ACT issues.
>>>
>> Sure. Until I can find more convincing evidence, I will make the changes as
>> requested.
> 
> Or you can point out the relevant part of the standard.
> 
Emm ,the DP HPG only specifies that an MST_ACT needs to be issued, and 
does not require validating its contents. This appears to be a 
Qualcomm‑specific hardware register, not a generic one.