[PATCH v8 5/6] drm/dp_mst: Add new quirk for Synaptics MST hubs

[Francis, David]

On 2019-08-26 2:05 p.m., David Francis wrote:
>> Synaptics DP1.4 hubs (BRANCH_ID 0x90CC24) do not
>> support virtual DPCD registers, but do support DSC.
>> The DSC caps can be read from the physical aux,
>> like in SST DSC. These hubs have many different
>> DEVICE_IDs.  Add a new quirk to detect this case.
>>
>> Cc: Lyude Paul <lyude at redhat.com>
>> Cc: Jani Nikula <jani.nikula at linux.intel.com>
>> Signed-off-by: David Francis <David.Francis at amd.com>
>> ---
>>  drivers/gpu/drm/drm_dp_helper.c | 2 ++
>>  include/drm/drm_dp_helper.h     | 7 +++++++
>>  2 files changed, 9 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
>> index 2cc21eff4cf3..fc39323e7d52 100644
>> --- a/drivers/gpu/drm/drm_dp_helper.c
>> +++ b/drivers/gpu/drm/drm_dp_helper.c
>> @@ -1270,6 +1270,8 @@ static const struct dpcd_quirk dpcd_quirk_list[] = {
>>        { OUI(0x00, 0x10, 0xfa), DEVICE_ID_ANY, false, BIT(DP_DPCD_QUIRK_NO_PSR) },
>>        /* CH7511 seems to leave SINK_COUNT zeroed */
>>        { OUI(0x00, 0x00, 0x00), DEVICE_ID('C', 'H', '7', '5', '1', '1'), false, BIT(DP_DPCD_QUIRK_NO_SINK_COUNT) },
>> +     /* Synaptics DP1.4 MST hubs can support DSC without virtual DPCD */
>> +     { OUI(0x90, 0xCC, 0x24), DEVICE_ID_ANY, true, BIT(DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD) },
>
>This seems to be generic OUI for Synaptics [1]. Could this cause us to
>cast the net too wide?
>
>Even if we check that it's DP_DPCD_REV >= 1.4 there's a good chance
>Synaptics is fixing this in the future and won't require the quirk.
>
>[1] https://aruljohn.com/mac/vendor/Synaptics
>
>
>Harry

It's not pretty, but
- Currently the net of "all Synaptics devices with rev>DP1.4" catches every device we care about and none we don't
- If a future Synaptics device supports virtual DPCD properly, we will check for that first and never resort to the workaround (see patch 6/6)
- We don't know any of the properties of any hypothetical future Synaptics hardware, so we can't create cases for them now

Also, received offline review from AMD MST DSC (Non-Linux) expert Wenjing Liu, giving me permission  o mark this patch
Reviewed-by: Wenjing Liu <Wenjing.Liu at amd.com>

>
>>  };
>>
>>  #undef OUI
>> diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
>> index 8364502f92cf..a1331b08705f 100644
>> --- a/include/drm/drm_dp_helper.h
>> +++ b/include/drm/drm_dp_helper.h
>> @@ -1434,6 +1434,13 @@ enum drm_dp_quirk {
>>         * The driver should ignore SINK_COUNT during detection.
>>         */
>>        DP_DPCD_QUIRK_NO_SINK_COUNT,
>> +     /**
>> +      * @DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD:
>> +      *
>> +      * The device supports MST DSC despite not supporting Virtual DPCD.
>> +      * The DSC caps can be read from the physical aux instead.
>> +      */
>> +     DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD,
>>  };
>>
>>  /**

________________________________________
From: Wentland, Harry <Harry.Wentland at amd.com>
Sent: August 26, 2019 3:05 PM
To: Francis, David; dri-devel at lists.freedesktop.org
Subject: Re: [PATCH v8 5/6] drm/dp_mst: Add new quirk for Synaptics MST hubs

On 2019-08-26 2:05 p.m., David Francis wrote:
> Synaptics DP1.4 hubs (BRANCH_ID 0x90CC24) do not
> support virtual DPCD registers, but do support DSC.
> The DSC caps can be read from the physical aux,
> like in SST DSC. These hubs have many different
> DEVICE_IDs.  Add a new quirk to detect this case.
>
> Cc: Lyude Paul <lyude at redhat.com>
> Cc: Jani Nikula <jani.nikula at linux.intel.com>
> Signed-off-by: David Francis <David.Francis at amd.com>
> ---
>  drivers/gpu/drm/drm_dp_helper.c | 2 ++
>  include/drm/drm_dp_helper.h     | 7 +++++++
>  2 files changed, 9 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index 2cc21eff4cf3..fc39323e7d52 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -1270,6 +1270,8 @@ static const struct dpcd_quirk dpcd_quirk_list[] = {
>       { OUI(0x00, 0x10, 0xfa), DEVICE_ID_ANY, false, BIT(DP_DPCD_QUIRK_NO_PSR) },
>       /* CH7511 seems to leave SINK_COUNT zeroed */
>       { OUI(0x00, 0x00, 0x00), DEVICE_ID('C', 'H', '7', '5', '1', '1'), false, BIT(DP_DPCD_QUIRK_NO_SINK_COUNT) },
> +     /* Synaptics DP1.4 MST hubs can support DSC without virtual DPCD */
> +     { OUI(0x90, 0xCC, 0x24), DEVICE_ID_ANY, true, BIT(DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD) },

This seems to be generic OUI for Synaptics [1]. Could this cause us to
cast the net too wide?

Even if we check that it's DP_DPCD_REV >= 1.4 there's a good chance
Synaptics is fixing this in the future and won't require the quirk.

[1] https://aruljohn.com/mac/vendor/Synaptics

Harry

>  };
>
>  #undef OUI
> diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
> index 8364502f92cf..a1331b08705f 100644
> --- a/include/drm/drm_dp_helper.h
> +++ b/include/drm/drm_dp_helper.h
> @@ -1434,6 +1434,13 @@ enum drm_dp_quirk {
>        * The driver should ignore SINK_COUNT during detection.
>        */
>       DP_DPCD_QUIRK_NO_SINK_COUNT,
> +     /**
> +      * @DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD:
> +      *
> +      * The device supports MST DSC despite not supporting Virtual DPCD.
> +      * The DSC caps can be read from the physical aux instead.
> +      */
> +     DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD,
>  };
>
>  /**
>