Re: [PATCH v6 2/2] drm: bridge: cdns-mhdp8546: Add no-hpd property

[Yashas D <y-d@ti.com>]

On 21/08/26 16:00, Tomi Valkeinen wrote:
> Hi,
> 
> On 02/08/2026 18:38, Yashas D wrote:
>> From: Rahul T R <r-ravikumar@ti.com>
>>
>> Add a 'no-hpd' boolean property to support boards where the HPD line
>> cannot be used for hotplug detection due to hardware limitations.
>>
>> On TI J721S2 EVMs, the DP0 HPD resistor is not populated from factory
>> (DNI), so the HPD signal is not physically connected to SoC pin AA24
>> by default which makes HPD unavailable but AA24 must be in DP0_HPD
>> mux mode for the MHDP firmware to operate.
>>
>> When this property is set, the driver uses auxiliary channel (AUX) DPCD
>> reads to detect monitor presence instead of hardware HPD signals. The
>> DRM framework polls the connection status via the .detect() callback,
>> providing hotplug detection without requiring the HPD pin.
>>
>> Valid use cases:
>> - HPD pin not routed to connector on PCB
>> - HPD signal muxed with another function on SoC
>> - Hardware designs where HPD cannot reliably detect monitor presence
>>
>> Signed-off-by: Rahul T R <r-ravikumar@ti.com>
>> Signed-off-by: Jayesh Choudhary <j-choudhary@ti.com>
>> Signed-off-by: Harikrishna Shenoy <h-shenoy@ti.com>
>> Signed-off-by: Yashas D <y-d@ti.com>
>> ---
>>   .../drm/bridge/cadence/cdns-mhdp8546-core.c   | 79 +++++++++++++++++--
>>   .../drm/bridge/cadence/cdns-mhdp8546-core.h   |  1 +
>>   2 files changed, 72 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c b/ 
>> drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
>> index 504a3186ebb3..ae9bbec855f3 100644
>> --- a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
>> +++ b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
>> @@ -53,6 +53,8 @@
>>   #include "cdns-mhdp8546-hdcp.h"
>>   #include "cdns-mhdp8546-j721e.h"
>> +static int cdns_mhdp_update_link_status(struct cdns_mhdp_device *mhdp);
>> +
>>   static void cdns_mhdp_bridge_hpd_enable(struct drm_bridge *bridge)
>>   {
>>       struct cdns_mhdp_device *mhdp = bridge_to_mhdp(bridge);
>> @@ -698,7 +700,9 @@ static int cdns_mhdp_fw_activate(const struct 
>> firmware *fw,
>>        * MHDP_HW_STOPPED happens only due to driver removal when
>>        * bridge should already be detached.
>>        */
>> -    cdns_mhdp_bridge_hpd_enable(&mhdp->bridge);
>> +
>> +    if (!mhdp->no_hpd)
>> +        cdns_mhdp_bridge_hpd_enable(&mhdp->bridge);
>>       spin_unlock(&mhdp->start_lock);
>> @@ -739,7 +743,13 @@ static void cdns_mhdp_fw_cb(const struct firmware 
>> *fw, void *context)
>>       spin_lock(&mhdp->start_lock);
>>       bridge_attached = mhdp->bridge_attached;
>>       spin_unlock(&mhdp->start_lock);
>> -    if (bridge_attached)
>> +
>> +    if (!bridge_attached)
>> +        return;
>> +
>> +    if (mhdp->no_hpd)
>> +        cdns_mhdp_update_link_status(mhdp);
>> +    else
>>           drm_bridge_hpd_notify(&mhdp->bridge, cdns_mhdp_detect(mhdp));
>>   }
>> @@ -788,9 +798,14 @@ static ssize_t cdns_mhdp_transfer(struct 
>> drm_dp_aux *aux,
>>           ret = cdns_mhdp_dpcd_read(mhdp, msg->address,
>>                         msg->buffer, msg->size);
>>           if (ret) {
>> -            dev_dbg(mhdp->dev,
>> -                "Failed to read DPCD addr %u\n",
>> -                msg->address);
>> +            if (mhdp->no_hpd)
>> +                dev_dbg(mhdp->dev,
>> +                    "Failed to read DPCD addr %u\n",
>> +                    msg->address);
>> +            else
>> +                dev_err(mhdp->dev,
>> +                    "Failed to read DPCD addr %u\n",
>> +                    msg->address);
> 
> Is this intentional? Earlier dev_dbg was used. Now the "normal" case is 
> dev_err. I think we can keep it as dev_dbg.
> 
>   Tomi
> 

Hi Tomi,
Earlier there was a discussion in v2 regarding the log level in 
cdns_mhdp_mailbox_read for the normal case and our case. I was trying to 
derive from that but keeping it dev_dbg for both the cases seems to be 
the best choice over here. Will send a next version and fix this.
Also please let me know if any more changes are required so that I can 
plan the next version.

Thanks,
Yashas D