Message-ID: <y262e67gi5f53objugljkpyc3lzdaqtw3b7qr4546btqo7ehu4@qp2orsf6xd7t>
Date: Wed, 28 May 2025 13:55:34 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Luca Ceresoli <luca.ceresoli@...tlin.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
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        Jernej Skrabec <jernej.skrabec@...il.com>,
        Jagan Teki <jagan@...rulasolutions.com>,
        Shawn Guo <shawnguo@...nel.org>, Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
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Subject: Re: [PATCH v4] drm/bridge: tc358767: convert to
 devm_drm_bridge_alloc() API

On Wed, May 28, 2025 at 11:29:36AM +0200, Luca Ceresoli wrote:
> This is the new API for allocating DRM bridges.
> 
> Converting this driver is a bit complex because the drm_bridge funcs
> pointer differs based on the bridge mode. So the current code does:
> 
>  * tc_probe()
>    * devm_kzalloc() private struct embedding drm_bridge
>    * call tc_probe_bridge_endpoint() which
>      * parses DT description into struct fields
>      * computes the mode
>      * calls different bridge init functions based on the mode
>        * each sets a different bridge.funcs pointer
> 
> The new API expects the funcs pointer to be known at alloc time, which does
> not fit in the current code structure.
> 
> Solve this by splitting tc_probe_bridge_endpoint() in two functions:
> 
>  * tc_probe_get_mode(), computing the mode without needing the private
>    driver structure
>  * tc_probe_bridge_endpoint(), only initializing the endpoints
> 
> So now the mode is known before allocation and so
> is the funcs pointer, while all other operations are still happening after
> allocation, directly into the private struct data, as they used to.
> 
> The new code flow is:
> 
>  * tc_probe()
>    * tc_probe_get_mode()
>      * parses DT description
>      * computes and returns the mode
>    * based onf the mode, pick the funcs pointer
>    * devm_drm_bridfge_alloc(..., funcs)
>    * call tc_probe_bridge_endpoint() which
>      * calls different bridge init functions based on the mode
>        * these don't set the funcs pointer, it was done by _alloc
> 
> This solution is chosen to minimize the changes in the driver logical code
> flow. The drawback is we now iterate twice over the endpoints during probe.
> 
> Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
> ---
> devm_drm_bridge_alloc() [0] is the new API to allocate and initialize a DRM
> bridge, and the only one supported from now on. It is the first milestone
> towards removal of bridges from a still existing DRM pipeline without
> use-after-free.
> 
> The steps in the grand plan [1] are:
> 
>  1. ➜ add refcounting to DRM bridges (struct drm_bridge)
>  2. handle gracefully atomic updates during bridge removal
>  3. avoid DSI host drivers to have dangling pointers to DSI devices
>  4. finish the hotplug bridge work, removing the "always-disconnected"
>     connector, moving code to the core and potentially removing the
>     hotplug-bridge itself (this needs to be clarified as points 1-3 are
>     developed)
> 
> This series is part of step 1 of the grand plan.
> 
> Current tasks in step 1 of the grand plan:
> 
>  A. ✔ add new alloc API and refcounting -> (now in drm-misc-next)
>  B. ➜ convert all bridge drivers to new API (this series)
>  C. … documentation, kunit tests (v1 under discussion)
>  D. after (B), add get/put to drm_bridge_add/remove() + attach/detech()
>  E. after (B), convert accessors; this is a large work and can be done
>     in chunks
>  F. debugfs improvements
> 
> More info about this series in the v2 cover [2].
> 
> Luca
> 
> [0] https://gitlab.freedesktop.org/drm/misc/kernel/-/commit/0cc6aadd7fc1e629b715ea3d1ba537ef2da95eec
> [1] https://lore.kernel.org/lkml/20250206-hotplug-drm-bridge-v6-0-9d6f2c9c3058@bootlin.com/t/#u
> [2] https://lore.kernel.org/lkml/20250424-drm-bridge-convert-to-alloc-api-v2-0-8f91a404d86b@bootlin.com/
> ---
> Changes in v4:
> - Removed patches already in drm-misc-next -> only 1 left
> - Improve commit message of patch 1
> - Link to v3: https://lore.kernel.org/all/20250509-drm-bridge-convert-to-alloc-api-v3-0-b8bc1f16d7aa@bootlin.com/
> 
> Changes in v3:
> - Fixed issues reported for some patches
> - Added review tags
> - Removed patches that have been applied
> - Added revert for the exynos patch, applied by mistake
> - Update cover with grand plan info and trim some of it
> - Updated bouncing e-mail address in Cc list
> - Link to v2: https://lore.kernel.org/lkml/20250424-drm-bridge-convert-to-alloc-api-v2-0-8f91a404d86b@bootlin.com/
> 
> Changes in v2:
> - Improved cover letter with link to commit adding devm_drm_bridge_alloc()
> - add review tags
> - fix bugs in zynqmp, vc4 patches
> - fix patch 1 error code checking
> - Link to v1: https://lore.kernel.org/r/20250407-drm-bridge-convert-to-alloc-api-v1-0-42113ff8d9c0@bootlin.com
> ---
> 

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>

-- 
With best wishes
Dmitry

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