[PATCH v2] drm/amd/display: Fix Apple dongle cannot be successfully detected

[Harry Wentland]

On 2019-11-22 1:33 a.m., Louis Li wrote:
> On Thu, Nov 21, 2019 at 08:47:50AM -0500, Kazlauskas, Nicholas wrote:
>> On 2019-11-21 8:40 a.m., Kazlauskas, Nicholas wrote:
>>> On 2019-11-21 3:31 a.m., Li, Ching-shih (Louis) wrote:
>>>> Hi reviewers,
>>>>
>>>> What is the review result for this patch? Customer is pushing on this
>>>> change to merge. TKS for your attention.
>>>>
>>>> BR,
>>>> Louis
>>>>
>>>> -----Original Message-----
>>>> From: Louis Li <Ching-shih.Li at amd.com>
>>>> Sent: Thursday, November 14, 2019 11:42 AM
>>>> To: amd-gfx at lists.freedesktop.org
>>>> Cc: Kazlauskas, Nicholas <Nicholas.Kazlauskas at amd.com>; Wentland, Harry
>>>> <Harry.Wentland at amd.com>; Li, Ching-shih (Louis) <Ching-shih.Li at amd.com>
>>>> Subject: [PATCH v2] drm/amd/display: Fix Apple dongle cannot be
>>>> successfully detected
>>>>
>>>> [Why]
>>>> External monitor cannot be displayed consistently, if connecting via
>>>> this Apple dongle (A1621, USB Type-C to HDMI).
>>>> By experiments, it is confirmed that the dongle needs 200ms at least to
>>>> be ready for communication, after it sets HPD signal high.
>>>>
>>>> [How]
>>>> When receiving HPD IRQ, delay 300ms at the beginning of handle_hpd_irq().
>>>> Then run the original procedure.
>>>> With this patch applied, the problem cannot be reproduced.
>>>> With other dongles, test results are PASS.
>>>> Test result is PASS to plug in HDMI cable while playing video, and the
>>>> video is being playing smoothly.
>>>> Test result is PASS after system resumes from suspend.
>>>>
>>>> Signed-off-by: Louis Li <Ching-shih.Li at amd.com>
>>>
>>> This is still a NAK from me since the logic hasn't changed from the first
>>> patch.
>>>
>>> Sleeps don't belong in IRQ handlers.
>>>
>>> Regards,
>>> Nicholas Kazlauskas
>>
>> Actually, this lives in the low IRQ context which means that it's already
>> been queued off to a work thread so it's safe to sleep.
>>
>> I'm not sure we want a general 300ms sleep (even by experiment you said that
>> it only needed 200ms) for all connectors.
>>
>> Nicholas Kazlauskas
>>
> 
> Yes, it is IRQ context. Safe to call sleep(). Moreover, in current driver,
> even udelay() is called in wait_for_training_aux_rd_interval() in the flow
> of handle_hpd_irq().
> 
> For 2nd question, of course not all connectors have this behavior.
> Based on real cases we ever dealt, some dongles like this, or some
> monitors driven by TCON, have same behavior. And no chance to read
> out anything to decide if delay is needed. This change does help
> to have our driver gain better compatibility. Truly this should be
> problem of dongles/monitors. We are not the only one to
> workaround such a problem. This change does not hurt other connects,
> and some other dongles are tested ok, e.g. HP/Huwai dongles, etc.
> 

I still don't like this change. It might impact other use cases, such as
SST-to-MST switching on MST displays.

Have you checked how Windows deals with this dongle and how the Windows
team solved this? Have you checked how other drivers (such as i915) deal
with this dongle?

Have you checked whether you can pass DP compliance with this change?

Harry

>>>
>>>> ---
>>>>   drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 5 +++++
>>>>   1 file changed, 5 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>> b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>> index 0aef92b7c037..5b844b6a5a59 100644
>>>> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>> @@ -1025,6 +1025,11 @@ static void handle_hpd_irq(void *param)
>>>>       struct drm_device *dev = connector->dev;
>>>>       enum dc_connection_type new_connection_type = dc_connection_none;
>>>> +    /* Some monitors/dongles need around 200ms to be ready for
>>>> communication
>>>> +     * after those devices drive HPD signal high.
>>>> +     */
>>>> +    msleep(300);
>>>> +
>>>>       /* In case of failure or MST no need to update connector status or
>>>> notify the OS
>>>>        * since (for MST case) MST does this in it's own context.
>>>>        */
>>>> -- 
>>>> 2.21.0
>>>>
>>>
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>>