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

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Message-ID: <CAHCN7xJxBrN5aQgvkV8LrqoTATinr0kFYKht2_YKqTF71UCoKw@mail.gmail.com>
Date:   Mon, 13 Mar 2023 06:08:05 -0500
From:   Adam Ford <aford173@...il.com>
To:     Sascha Hauer <s.hauer@...gutronix.de>
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 Mon, Mar 13, 2023 at 3:51 AM Sascha Hauer <s.hauer@...gutronix.de> wrote:
>
> 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.

I thought about that, but on the Nano, the video_pll is several layers up.

video_pll -> video_pll_bypass -> disp_pixel -> disp_pixel_clk

Do I just set that flag for each of these?

adam
>
> Sascha
>
> --
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