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

Martyn Welch
2026-03-26T17:15:25Z

On 20/03/2026 10:46, Luca Ceresoli wrote:
> 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!
>

Tested-by: Martyn Welch <martyn.welch@collabora.com>

Using Ezurio Nitrogen8M Plus ENC Carrier Board with i.MX88MP SOM and
1280x800 HDMI display.

> == 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@bootlin.com/#t
>
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> ---
> 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,