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

[Tomi Valkeinen]

Hi,

On 02/08/2026 18:38, Yashas D wrote:
> From: Rahul T R <r-ravikumar@xxxxxx>
>
> 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@xxxxxx>
> Signed-off-by: Jayesh Choudhary <j-choudhary@xxxxxx>
> Signed-off-by: Harikrishna Shenoy <h-shenoy@xxxxxx>
> Signed-off-by: Yashas D <y-d@xxxxxx>
> ---
>   .../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