linux-kernel - Re: [PATCH v2 2/2] drm/bridge: add support for lontium lt9211c bridge

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Message-ID: <aTpVkaMJNhTb2zyk@hu-botlagun-hyd.qualcomm.com>
Date: Thu, 11 Dec 2025 10:54:33 +0530
From: Gopi Botlagunta <venkata.botlagunta@....qualcomm.com>
To: Marek Vasut <marek.vasut@...lbox.org>
Cc: Nilesh Laad <nilesh.laad@....qualcomm.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>, Marek Vasut <marex@...x.de>,
        dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, venkata.valluru@....qualcomm.com,
        jessica.zhang@....qualcomm.com, Yi Zhang <zhanyi@....qualcomm.com>
Subject: Re: [PATCH v2 2/2] drm/bridge: add support for lontium lt9211c bridge

On Fri, Nov 07, 2025 at 04:11:15PM +0100, Marek Vasut wrote:
> On 11/7/25 2:52 PM, Nilesh Laad wrote:
> > On Fri, Nov 07, 2025 at 02:20:58PM +0100, Marek Vasut wrote:
> > > On 11/7/25 2:02 PM, Nilesh Laad wrote:
> > > > From: Yi Zhang <zhanyi@....qualcomm.com>
> > > > 
> > > > LT9211c is a Single/Dual-Link DSI/LVDS or Single DPI input to
> > > > Single-link/Dual-Link DSI/LVDS or Single DPI output bridge chip.
> > > > Add support for DSI to LVDS bridge configuration.
> > > How does this differ from existing drivers/gpu/drm/bridge/lontium-lt9211.c ?
> > > Can existing lt9211 driver be extended instead ? If not, why ? Details
> > > please ...
> > LT9211 and LT9211C differ completely in register programming sequences.
> > Even lontium mentioned that register configuration are different for lt9211 and lt9211c.
> 
> Lontium seems to often suggest, that users should use their provided
> register patches without thinking about the content at all.
> 
> Do you have access to the register documentation, and can you compare LT9211
> and LT9211C register layout? Are they identical or do they differ?

We don’t have access to register documentation. Based on Lontium provided reference code, register sequences differ.
We are seeing this limited difference, we plan to extend the existing lontium‑lt9211 driver to support LT9211C. Will share a patch with these changes.

> > Nearly every function would require duplicated logic with if (chip_type) branching,
> > as register sequence are completely different.
> > Having both sequences in single file is not looking good, hence want to merge as separate driver.
> 
> Can we somehow use regmap_register_patch() and register patches in driver
> data to avoid duplication ?

Using regmap_register_patch() would not reduce duplication. The bridge configuration between LT9211 and LT9211C are different, leaving minimal common base and still requiring separate calculation logic and control flow.

> -- 
> Best regards,
> Marek Vasut