linux-kernel - Re: [PATCH 5/6] drm: bridge: samsung-dsim: Support non-burst mode

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Message-ID: <88e53197-2819-c068-eba6-a218a19d8d15@denx.de>
Date:   Mon, 17 Apr 2023 22:07:56 +0200
From:   Marek Vasut <marex@...x.de>
To:     Adam Ford <aford173@...il.com>
Cc:     dri-devel@...ts.freedesktop.org, m.szyprowski@...sung.com,
        aford@...conembedded.com, Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Inki Dae <inki.dae@...sung.com>,
        Jagan Teki <jagan@...rulasolutions.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>,
        Daniel Vetter <daniel@...ll.ch>,
        Frieder Schrempf <frieder.schrempf@...tron.de>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Lucas Stach <l.stach@...gutronix.de>
Subject: Re: [PATCH 5/6] drm: bridge: samsung-dsim: Support non-burst mode

On 4/17/23 13:57, Adam Ford wrote:
> On Sun, Apr 16, 2023 at 5:13 PM Marek Vasut <marex@...x.de> wrote:
>>
>> On 4/15/23 12:41, Adam Ford wrote:
>>> The high-speed clock is hard-coded to the burst-clock
>>> frequency specified in the device tree.  However, when
>>> using devices like certain bridge chips without burst mode
>>> and varying resolutions and refresh rates, it may be
>>> necessary to set the high-speed clock dynamically based
>>> on the desired pixel clock for the connected device.
>>
>> The link rate negotiation should happen internally between the nearest
>> bridge and DSIM, so please add that to DRM core instead of hacking
>> around it by tweaking the HS clock again.
> 
> I thought you tried to add something like this before and had some resistance.

Yes, all my attempts were rejected by a single reviewer. I suspended my 
efforts in that area for now.

> The Pixel clock is set by the bridge already without any new code
> added to the DRM core..  I am just reading that value that's there,
> and setting the clock accordingly.  I don't see how this is a hack.

Assume you have a DSI-to-HDMI bridge attached to your DSIM bridge, it 
operates in non-burst mode, like ADV7533 . How would you configure the 
HS clock rate for such a bridge in DT ? (hint: you cannot, because the 
required clock comes from the EDID, which may not be available just yet)