Re: [PATCH v4 0/3] drm: simplify support for transparent DRM bridges

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Dmitry,

Thank you for the patches.

On Thu, Aug 17, 2023 at 05:55:13PM +0300, Dmitry Baryshkov wrote:
> Supporting DP/USB-C can result in a chain of several transparent
> bridges (PHY, redrivers, mux, etc). This results in drivers having
> similar boilerplate code for such bridges.

What do you mean by transparent bridge here ? Bridges are a DRM concept,
and as far as I can tell, a PHY isn't a bridge. Why does it need to be
handled as one, especially if it's completely transparent ?

> Next, these drivers are susceptible to -EPROBE_DEFER loops: the next
> bridge can either be probed from the bridge->attach callback, when it is
> too late to return -EPROBE_DEFER, or from the probe() callback, when the
> next bridge might not yet be available, because it depends on the
> resources provided by the probing device.

Can't device links help avoiding defer probing in those cases ?

> Last, but not least, this results in the the internal knowledge of DRM
> subsystem slowly diffusing into other subsystems, like PHY or USB/TYPEC.

Why so ? The PHY subsystem should provide a PHY, without considering
what subsystem it will be used by. This patch series seems to me to
actually create this DRM dependency in other subsystems, which I don't
think is a very good idea. Resources should be registered in their own
subsystem with the appropriate API, not in a way that is tied to a
particular consumer.

> To solve all these issues, define a separate DRM helper, which creates
> separate aux device just for the bridge. During probe such aux device
> doesn't result in the EPROBE_DEFER loops. Instead it allows the device
> drivers to probe properly, according to the actual resource
> dependencies. The bridge auxdevs are then probed when the next bridge
> becomes available, sparing drivers from drm_bridge_attach() returning
> -EPROBE_DEFER.

I'm not thrilled :-( Let's discuss the questions above first.

> Proposed merge strategy: immutable branch with the drm commit, which is
> then merged into PHY and USB subsystems together with the corresponding
> patch.
> 
> Changes since v3:
>  - Moved bridge driver to gpu/drm/bridge (Neil Armstrong)
>  - Renamed it to aux-bridge (since there is already a simple_bridge driver)
>  - Made CONFIG_OF mandatory for this driver (Neil Armstrong)
>  - Added missing kfree and ida_free (Dan Carpenter)
> 
> Changes since v2:
>  - ifdef'ed bridge->of_node access (LKP)
> 
> Changes since v1:
>  - Added EXPORT_SYMBOL_GPL / MODULE_LICENSE / etc. to drm_simple_bridge
> 
> Dmitry Baryshkov (3):
>   drm/bridge: add transparent bridge helper
>   phy: qcom: qmp-combo: switch to DRM_AUX_BRIDGE
>   usb: typec: nb7vpq904m: switch to DRM_AUX_BRIDGE
> 
>  drivers/gpu/drm/bridge/Kconfig            |   9 ++
>  drivers/gpu/drm/bridge/Makefile           |   1 +
>  drivers/gpu/drm/bridge/aux-bridge.c       | 132 ++++++++++++++++++++++
>  drivers/phy/qualcomm/Kconfig              |   2 +-
>  drivers/phy/qualcomm/phy-qcom-qmp-combo.c |  44 +-------
>  drivers/usb/typec/mux/Kconfig             |   2 +-
>  drivers/usb/typec/mux/nb7vpq904m.c        |  44 +-------
>  include/drm/bridge/aux-bridge.h           |  19 ++++
>  8 files changed, 167 insertions(+), 86 deletions(-)
>  create mode 100644 drivers/gpu/drm/bridge/aux-bridge.c
>  create mode 100644 include/drm/bridge/aux-bridge.h

-- 
Regards,

Laurent Pinchart

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