Re: [PATCH v5 0/2] Add no-hpd property to the cadence bridge

On 30/07/26 21:28, Dominik Haller wrote:
> On Mi, 2026-07-29 at 13:54 +0530, Yashas D wrote:
>>
>>
>> On 29/07/26 04:07, Dominik Haller wrote:
>>> On Di, 2026-07-28 at 18: 23 +0530, Yashas D wrote: > This series
>>> adds
>>> 'no-hpd' device tree property support to the Cadence > MHDP8546
>>> bridge
>>> driver for boards where the HPD line cannot be used > for > hotplug
>>> detection. > >
>>>
>>>
>>> On Di, 2026-07-28 at 18:23 +0530, Yashas D wrote:
>>>> This series adds 'no-hpd' device tree property support to the
>>>> Cadence
>>>> MHDP8546 bridge driver for boards where the HPD line cannot be
>>>> used
>>>> for
>>>> hotplug detection.
>>>>
>>>> On TI J721S2 EVMs, the HPD signal is routed to SoC pin AA24
>>>> (MCASP1_ACLKX/DP0_HPD). This pin is muxed with the McASP1 audio
>>>> bit
>>>> clock; selecting DP0_HPD breaks audio. There is no alternative
>>>> pin
>>>> carrying the HPD signal which makes it impossible to use HPD
>>>> without loosing the audio capabilities.
>>> Hi Yashas,
>>>
>>> this description is not correct. On the J721S2 EVM the pin AA24
>>> MCASP1_ACLKX is not used for audio.
>>> The pin is just floating unless this "GESI"-board with an
>>> additional
>>> ethernet interface is connect. AA24 is used as as
>>> MCASP1_ACLKX.RGMII1_RD0 in this case.
>>> See arch/arm64/boot/dts/ti/k3-j721s2-evm-gesi-exp-board.dtso
>>>
>>> Also the AA24 pin needs to be configured as HPD pin (muxmode 3)
>>> otherwise the mhdp firmware will not turn on the bridge.
>>>
>>> Dominik
>>>
>>>
>>
>> Hi Dominik,
>>
>> You are correct, I apologise for the error. The SoC PAD mux conflict
>> on AA24 is with RGMII1_RD0 (GESI ethernet expansion board), not
>> MCASP1
>> audio. I will fix the commit message.
>>
>> The full picture on J721S2X-PM1-SOM rev E2:
>>
>> - The HPD resistor is DNI from factory, so AA24 is not physically
>>     connected to the DP connector HPD line by default. However AA24
>> must
>>     still be configured in DP0_HPD mux mode (mode 3) for the MHDP
>> firmware
>>     to operate at all — configuring it in any other mode prevents the
>> bridge from loading.
>>
>> - When we physically connected HPD by populating the resistor, the E2
>>     board failed to boot with a monitor attached. The HPD line leaks
>> voltage
>>     from the monitor into the SoC while powered off, causing a PMIC
>> error.
>>     This was fixed in rev E3+ with an HPD buffer. Our boards are rev
>> E2.
>>
>> The audio conflict in earlier message versions was our mistake. It
>> came from investigating whether an alternative routing path through a
>> board-level signal MUX chip (MUX1/MUX2) on the SOM could expose a
>> GPIO
>> for HPD — changing that MUX's port selection would have affected
>> audio.
>> That is unrelated to the SoC PAD mux and should not have been
>> mentioned.
>>
>> I will fix the message and send a new version.
> Hi Yashas,
> 
> please also update the commit description of the actual patch 2/2 as it
> implies at the moment that you can use the AA24 pin for other functions
> while using the display port.
> 
> Thanks
> 
> Dominik
>>
>> Thanks,
>> Yashas D
>>

Hi Dominik,
Have updated it in v6.

Thanks,
Yashas D
>>>> When 'no-hpd' is set, DRM_BRIDGE_OP_HPD is omitted so the
>>>> framework
>>>> falls back to polling .detect() every ~10 seconds. Monitor
>>>> presence
>>>> is
>>>> determined via AUX DPCD reads instead of firmware HPD status
>>>> registers.
>>>> The .detect() callback drives cdns_mhdp_update_link_status() on
>>>> each
>>>> poll to keep mhdp->plugged current. At attach time, the driver
>>>> waits
>>>> for firmware to be ready before performing the initial AUX poll
>>>> since
>>>> no interrupt will trigger it.
>>>>
>>>> Changes since v4:
>>>> - Corrected the dt-binding Subject
>>>>
>>>> Link to v4:
>>>> https://urldefense.com/v3/__https://lore.kernel.org/
>>>>
>>> all/[email protected]/__;!!G3vK!
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>>> urldefense.com%2Fv3%2F__https%3A%2F%2Flore.kernel.org%2Fall%2F20260
>>> 630102610.1849902-1-y-
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>>> ZT9JSyfcyMp%2BDjQPDUC%2F0%3D&reserved=0>
>>>>
>>>> Rahul T R (2):
>>>>    dt-bindings: display: bridge: cdns,mhdp8546: Add no-hpd
>>>> property to
>>>>      the cadence bridge
>>>>    drm: bridge: cdns-mhdp8546: Add no-hpd property
>>>>
>>>>   .../display/bridge/cdns,mhdp8546.yaml         | 17 ++++++
>>>>   .../drm/bridge/cadence/cdns-mhdp8546-core.c   | 58
>>>> ++++++++++++++++-
>>>> --
>>>>   .../drm/bridge/cadence/cdns-mhdp8546-core.h   |  1 +
>>>>   3 files changed, 69 insertions(+), 7 deletions(-)
>>>>
>>>
>>
>