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

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
>
> > > 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!
> > XKetSJhTJzSznMTBeoww63bvXSPD3xrWstRrf4H6AhnrM5IjIvNFa9uHVVWEYPEoe0G
> > NrZXGb8IF$
> > <https://eur02.safelinks.protection.outlook.com/?url=https%3A%2F%2F
> > urldefense.com%2Fv3%2F__https%3A%2F%2Flore.kernel.org%2Fall%2F20260
> > 630102610.1849902-1-y-
> > d%40ti.com%2F__%3B!!G3vK!XKetSJhTJzSznMTBeoww63bvXSPD3xrWstRrf4H6Ah
> > nrM5IjIvNFa9uHVVWEYPEoe0GNrZXGb8IF%24&data=05%7C02%7Cd.haller%40phy
> > tec.de%7Ce1e53182268a4df81d1908deed4ada4b%7Ce609157c80e2446d9be39c9
> > 9c2399d29%7C0%7C0%7C639209102909756678%7CUnknown%7CTWFpbGZsb3d8eyJF
> > bXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWF
> > pbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=2TH3dx%2FirnEKUcHFu9vOz%2F
> > 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(-)
> > >
> >
>