linux-kernel - Re: [PATCH 00/13] imx8mp: first clock propagation attempt (for LVDS)

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Date:   Tue, 19 Sep 2023 09:39:58 +0200
From:   Maxime Ripard <mripard@...nel.org>
To:     Benjamin Bara <bbara93@...il.com>
Cc:     Frank Oltmanns <frank@...manns.dev>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.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>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Abel Vesa <abelvesa@...nel.org>, Peng Fan <peng.fan@....com>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org,
        Benjamin Bara <benjamin.bara@...data.com>,
        Adam Ford <aford173@...il.com>,
        Lucas Stach <l.stach@...gutronix.de>
Subject: Re: [PATCH 00/13] imx8mp: first clock propagation attempt (for LVDS)

On Mon, Sep 18, 2023 at 08:05:48PM +0200, Benjamin Bara wrote:
> Hi Frank!
> 
> On Mon, 18 Sept 2023 at 19:24, Frank Oltmanns <frank@...manns.dev> wrote:
> > On 2023-09-18 at 00:39:56 +0200, Benjamin Bara <bbara93@...il.com> wrote:
> > Thank you very much for including me in the discussion. If I understood
> > Maxime correctly, your proposal is close to what he was suggesting in
> > the discussion you referenced. Unfortunately, it doesn't cover the
> > rounding aspect (which you also mentioned in your cover letter and the
> > description for clk_detect_unintended_rate_changes in patch 7. I've been
> > pondering the last three weeks how to find a good solution to this
> > problem, but so far haven't found any.
> 
> I think if we stick to the idea of always enforcing the exact "typical
> rate", we cannot avoid physically impossible cases. IMHO, it might make
> sense to add a set_rate() function with a "timing_entry" (e.g. used by
> display_timing.h[1]) to the clock API, which gives a suggestion but also
> defines the "real" boundaries. This would provide a shared parent PLL
> more freedom to provide a satisfying rate for all its children.

It's definitely something we should do, and I've wanted to do that for a
while.

The clock rate is not the only thing we can change though. The usual
trick is to modify the blanking areas to come up with a rate that
matches what the hardware can provide without modifying the framerate.

It belongs more in a KMS helper

Maxime

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