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

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Message-ID: <CAJpcXm7nrLzbDmNVCACy3ycBnhWY7HxWhCwroOCF-+GfMT_A9w@mail.gmail.com>
Date:   Mon, 18 Sep 2023 20:05:48 +0200
From:   Benjamin Bara <bbara93@...il.com>
To:     Frank Oltmanns <frank@...manns.dev>
Cc:     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>,
        Maxime Ripard <mripard@...nel.org>, 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)

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.

Regards
Benjamin

[1] https://elixir.bootlin.com/linux/v6.5.3/source/include/video/display_timing.h#L64