Re: [PATCH 0/8] drm/mxsfb/lcdif: use DRM_BRIDGE_ATTACH_NO_CONNECTOR and the bridge-connector

[Alexander Stein]

Hi Luca,

Am Freitag, 20. März 2026, 11:46:11 CET schrieb Luca Ceresoli:
> This series modernizes the i.mx8mp LCDIF driver to use the
> bridge-connector, which is the current best practice in DRM.
> 
> == Call for testing on i.MX8MP boards (especially those using HDMI)!
> 
> This series applies changes to how video output devices are probed on
> i.MX8MP, especially those using HDMI. Even though I have put care in not
> breaking anything, there could potentially be pitfalls I haven't realized,
> causing regressions on existing boards.
> 
> I have thus added in Cc all developers which appeared active on dts files
> for imx8mp boards involving video. I would appreciate testing on as many
> boards as possible, along with a Tested-by tag, or a report about any
> issues encountered.
> 
> Thanks in advance to all testers!

For the whole series:
Tested-by: Alexander Stein <alexander.stein@xxxxxxxxxxxxxxx> # TQMa8MPxL/MBa8MPxL

Because this platform falls into type A of patch 7
DRM_IMX8MP_DW_HDMI_BRIDGE_CONNECTOR_FIXUP has been disabled for me.

Best regards,
Alexander

> 
> == Review recommendation
> 
> I recommend reviewing patches in this order to be understood more
> effectively:
> 
>  * Cover letter
>  * Patches 1-5 are small preliminary cleanups/improvements
>  * Patch 8 is the goal of this series, but would not work alone
>  * Patch 7 this lets patch 8 work; but in turn it can't work alone
>  * Patch 6 lets patch 7 work
> 
> == Series description
> 
> This series is not strictly related to DRM bridge hotplug, it is rather a
> preparation step. Introducing hotplug would need two different approaches:
> one for the new way, for drivers using bridge-connector and
> DRM_BRIDGE_ATTACH_NO_CONNECTOR, another for drivers using the "old, legacy
> way" where the last bridge is supposed to instantiate the
> drm_connector. Hotplug is complicated enough in one case, so it makes sense
> to only support the new way.
> 
> The hardware I'm working on is an i.MX8MP, whose LCDIF driver is still
> using the old way. So this series converts to the new way as a preparation
> step.
> 
> Patch 8 does the conversion, which is simple. However this would introduce
> a regression on some boards. Here's why:
> 
> There are 3 instances of the LCDIF in i.MX8MP:
> 
>  * LCDIF1, driving the DSI output
>  * LCDIF2, driving the LVDS output
>  * LCDIF3, driving the HDMI output
> 
> The device drivers of peripherals connected to LCDIF1 and LCDIF2 already
> support the DRM_BRIDGE_ATTACH_NO_CONNECTOR flag. So far so good.
> 
> LCDIF3 is more tricky. The HDMI pipeline is:
> 
>   LCDIF3 -> fsl,imx8mp-hdmi-pvi -> fsl,imx8mp-hdmi-tx -> HDMI connector
> 
> The fsl,imx8mp-hdmi-tx (hdmi-tx) component supports both cases (with or
> without DRM_BRIDGE_ATTACH_NO_CONNECTOR), but in the
> DRM_BRIDGE_ATTACH_NO_CONNECTOR case it does not create a drm_connector,
> thus preventing the creation of the pipeline. To make it work a connector
> must be created, and the way to do so is describing the connector in the
> device tree (compatible = "hdmi-connector"), so the display-driver will add
> the connector along with a wrapping bridge.
> 
> Unfortunately not all device trees in mainline have an hdmi-connector
> node. Adding one is easy, but would break existing hardware upgrading to a
> newer kernel without upgrading the device tree blob. This is addressed by
> patch 7 reusing an existing approach to add such a node to the live device
> tree at init time using a device tree overlay for boards which don't have
> one.
> 
> Finally, patch 7 cannot work alone because of a bad interaction between
> devlink and device tree overlays. Patch 6 solves that.
> 
> == Grand plan
> 
> This is part of the work to support hotplug of DRM bridges. The grand plan
> was discussed in [0].
> 
> Here's the work breakdown (➜ marks the current series):
> 
>  1. … add refcounting to DRM bridges struct drm_bridge,
>       based on devm_drm_bridge_alloc()
>     A. ✔ add new alloc API and refcounting (v6.16)
>     B. ✔ convert all bridge drivers to new API (v6.17)
>     C. ✔ kunit tests (v6.17)
>     D. ✔ add get/put to drm_bridge_add/remove() + attach/detach()
>          and warn on old allocation pattern (v6.17)
>     E. … add get/put on drm_bridge accessors
>        1. ✔ drm_bridge_chain_get_first_bridge(), add cleanup action (v6.18)
>        2. ✔ drm_bridge_get_prev_bridge() (v6.18)
>        3. ✔ drm_bridge_get_next_bridge() (v6.19)
>        4. ✔ drm_for_each_bridge_in_chain() (v6.19)
>        5. ✔ drm_bridge_connector_init (v6.19)
>        6. … protect encoder bridge chain with a mutex
>        7. … of_drm_find_bridge
>           a. ✔ add of_drm_get_bridge() (v7.0),
> 	       convert basic direct users (v7.0-v7.1)
> 	  b. ✔ convert direct of_drm_get_bridge() users, part 2 (v7.0)
> 	  c. ✔ convert direct of_drm_get_bridge() users, part 3 (v7.0)
> 	  d. ✔… convert direct of_drm_get_bridge() users, part 4
> 	        (some v7.1, some pending)
> 	  e.   convert bridge-only drm_of_find_panel_or_bridge() users
>        8. drm_of_find_panel_or_bridge, *_of_get_bridge
>        9. ✔ enforce drm_bridge_add before drm_bridge_attach (v6.19)
>     F. ✔ debugfs improvements
>        1. ✔ add top-level 'bridges' file (v6.16)
>        2. ✔ show refcount and list lingering bridges (v6.19)
>  2. … handle gracefully atomic updates during bridge removal
>     A. ✔ Add drm_bridge_enter/exit() to protect device resources (v7.0)
>     B. … protect private_obj removal from list
>     C. ✔ Add drm_bridge_clear_and_put() (v7.1)
>  3. … DSI host-device driver interaction
>  4. ✔ removing the need for the "always-disconnected" connector
>  5. ➜ Migrate i.MX LCDIF driver to bridge-connector
>  6.   DRM bridge hotplug
>     A.   Bridge hotplug management in the DRM core
>     B.   Device tree description
> 
> [0] https://lore.kernel.org/lkml/20250206-hotplug-drm-bridge-v6-0-9d6f2c9c3058@xxxxxxxxxxx/#t
> 
> Signed-off-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
> ---
> Luca Ceresoli (8):
>       drm/mxsfb/lcdif: simplify remote pointer management using __free
>       drm/mxsfb/lcdif: don't unnecessarily loop over ports
>       drm/mxsfb/lcdif: use dev_err_probe() consistently in lcdif_attach_bridge
>       drm/bridge: dw-hdmi: document the output_port field
>       drm/bridge: dw-hdmi: warn on unsupported attach combination
>       drm/bridge: dw-hdmi: move next_bridge lookup to attach time
>       drm/bridge: imx8mp-hdmi-tx: add an hdmi-connector when missing using a DT overlay at boot time
>       drm/mxsfb/lcdif: use DRM_BRIDGE_ATTACH_NO_CONNECTOR and the bridge-connector
> 
>  drivers/gpu/drm/bridge/imx/Kconfig                 | 17 +++++
>  drivers/gpu/drm/bridge/imx/Makefile                |  2 +
>  .../bridge/imx/imx8mp-hdmi-tx-connector-fixup.c    | 60 +++++++++++++++
>  .../bridge/imx/imx8mp-hdmi-tx-connector-fixup.dtso | 38 ++++++++++
>  drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx.c        |  1 +
>  drivers/gpu/drm/bridge/synopsys/dw-hdmi.c          | 45 ++++-------
>  drivers/gpu/drm/mxsfb/Kconfig                      |  1 +
>  drivers/gpu/drm/mxsfb/lcdif_drv.c                  | 88 +++++++++-------------
>  include/drm/bridge/dw_hdmi.h                       |  5 ++
>  9 files changed, 174 insertions(+), 83 deletions(-)
> ---
> base-commit: 8402cf4fc8f8d5756dc81cf9fda1dccdb3622634
> change-id: 20260306-drm-lcdif-dbanc-83dd948327de
> 
> Best regards,
> 


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