linux-kernel - Re: MXSFB and Video PLL clock on i.MX8M Mini/Nano Question

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Date:   Mon, 13 Mar 2023 09:51:05 +0100
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Adam Ford <aford173@...il.com>
Cc:     arm-soc <linux-arm-kernel@...ts.infradead.org>,
        Marek Vasut <marex@...x.de>, Stefan Agner <stefan@...er.ch>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Shawn Guo <shawnguo@...nel.org>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jagan Teki <jagan@...rulasolutions.com>
Subject: Re: MXSFB and Video PLL clock on i.MX8M Mini/Nano Question

On Sun, Mar 12, 2023 at 02:28:45PM -0500, Adam Ford wrote:
> I am trying to work through a series that was submitted for enabling
> the DSI on the i.MX8M Mini and Nano.  I have extended this series to
> route the DSI to an HDMI bridge, and I am able to get several
> resolutions to properly sync on my monitor.  However, there are also a
>  bunch that appear on the list when I run modetest that do not sync on
> my monitor.
> 
> When running some debug code, it appears that it's related to the
> clocking of the MXSFB driver.
> 
> From what I can tell, the MSXFB driver attempts to set the clock based
> on the desired resolution and refresh rate.  When the default
> VIDEO_PLL clock is set to 594MHz, many of the resolutions that cleanly
> divide from the 594MHz clock appear to sync with my monitor.  However,
> in order to get other resolutions to appear, I have to manually change
> the device tree to set VIDEO_PLL to a different clock rate so MSXFB
> can use it.  Unfortunately, that breaks the resolutions that used to
> work.
> 
> I threw together a hack into the MXSFB driver which adds a new
> optional clock to the MSXFB driver.  When I pass VIDEO_PLL to this
> driver, it can automatically set the clock rate to match that of
> whatever the desired clock is, and I can get many more resolutions to
> appear.
> Another advantage of this is that the Video_PLL can be the minimum
> speed needed for a given rate instead of setting a higher rate, then
> dividing it down.

Isn't it possible to add the CLK_SET_RATE_PARENT flag to the pixel
clock? That's what i.MX6sx and i.MX7 do.

Sascha

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