MXSFB and Video PLL clock on i.MX8M Mini/Nano Question

[Sascha Hauer]

On Mon, Mar 13, 2023 at 06:08:05AM -0500, Adam Ford wrote:
> On Mon, Mar 13, 2023 at 3:51 AM Sascha Hauer <s.hauer at pengutronix.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?

Yes, that's what I would suggest. I don't know the i.MX8M clock tree
very well, so I don't know which other clocks might be influenced when
the video_pll clock changes its rate. But if you have to change the PLL
rate anyway it shouldn't make a difference if you change it directly
or if you let the rate change propagate up from disp_pixel_clk.

Sascha


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