Re: [PATCH v2 2/3] drm/msm/disp/dpu1: add helper to know if display is pluggable

[Abhinav Kumar <quic_abhinavk@quicinc.com>]

On 11/16/2022 7:18 AM, Dmitry Baryshkov wrote:
> On 16/11/2022 18:11, Abhinav Kumar wrote:
>>
>>
>> On 11/16/2022 7:08 AM, Dmitry Baryshkov wrote:
>>> On 16/11/2022 17:30, Kalyan Thota wrote:
>>>> Since DRM encoder type for few encoders can be similar
>>>> (like eDP and DP) find out if the interface supports HPD
>>>> from encoder bridge to differentiate between builtin
>>>> and pluggable displays.
>>>>
>>>> Changes in v1:
>>>> - add connector type in the disp_info (Dmitry)
>>>> - add helper functions to know encoder type
>>>> - update commit text reflecting the change
>>>>
>>>> Changes in v2:
>>>> - avoid hardcode of connector type for DSI as it may not be true 
>>>> (Dmitry)
>>>> - get the HPD information from encoder bridge
>>>>
>>>> Signed-off-by: Kalyan Thota <quic_kalyant@quicinc.com>
>>>> ---
>>>>   drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c | 16 ++++++++++++++++
>>>>   drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.h |  6 ++++++
>>>>   2 files changed, 22 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c 
>>>> b/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
>>>> index 9c6817b..be93269 100644
>>>> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
>>>> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
>>>> @@ -15,6 +15,7 @@
>>>>   #include <drm/drm_crtc.h>
>>>>   #include <drm/drm_file.h>
>>>>   #include <drm/drm_probe_helper.h>
>>>> +#include <drm/drm_bridge.h>
>>>>   #include "msm_drv.h"
>>>>   #include "dpu_kms.h"
>>>> @@ -217,6 +218,21 @@ static u32 dither_matrix[DITHER_MATRIX_SZ] = {
>>>>       15, 7, 13, 5, 3, 11, 1, 9, 12, 4, 14, 6, 0, 8, 2, 10
>>>>   };
>>>> +bool dpu_encoder_is_pluggable(struct drm_encoder *encoder)
>>>> +{
>>>> +    struct drm_bridge *bridge;
>>>> +    int ops = 0;
>>>> +
>>>> +    if (!encoder)
>>>> +        return false;
>>>> +
>>>> +    /* Get last bridge in the chain to determine pluggable state */
>>>> +    drm_for_each_bridge_in_chain(encoder, bridge)
>>>> +        if (!drm_bridge_get_next_bridge(bridge))
>>>> +            ops = bridge->ops;
>>>> +
>>>> +    return ops & DRM_BRIDGE_OP_HPD;
>>>
>>> No. This is not what you should be checking (hint: polled connectors 
>>> also can be pluggable).
>>>
>>> Please check the type of the actual connector connected to this encoder.
>>>
>>
>> Even if we check the connector type as DSI or eDP that does not 
>> necessarily mean its built-in.
>>
>> We can even use DSI or eDP as a pluggable display.
> 
> Well, I don't think so. eDP and DSI connectors are not pluggable per 
> design. One can use them so, but they are not thought to be used this 
> way. Unlike e.g. HDMI, DP, VGA, etc.
> 

We have had many products where we used HDMI as the primary display 
where the HPD line was disconnected in the design, so now if we 
generalize all HDMI connectors to be pluggable we can never enable color 
management on those even though DSI is not even used in that product.

Thats why I felt we should rely on the HPD_OPS as that way we know that 
it will be set only if HPD will be used.

Wouldnt it be just better to also check polling displays to complete 
this check? Is there a way to do it?

> I would say LVDS, eDP, DSI, DPI and SPI can be assumed to be constantly 
> plugged.
> 
> Compare this with Composite, SVIDEO, 9PinDIN, TV. They can be assumed to 
> be external even if they do not have the HPD (or even polling). And 
> these connectors usually don't have it.
> 
>>
>> Thats why we thought of this check.
>>