i.MX8MP IMX-LCDIF Underrun Question(s)

[Maxime Ripard]

On Fri, Sep 06, 2024 at 08:03:38AM GMT, Peng Fan wrote:
> > Subject: i.MX8MP IMX-LCDIF Underrun Question(s)
> > 
> > I have been testing various settings on the HDMI out of the i.MX8MP.
> > 
> > I noticed that sometimes my monitor would not sync, but sometimes it
> > would on the same resolution/refresh rate.  Frieder noted the LCDIF
> > was sometimes underflowing, so read up on it a little bit.
> > 
> > In the comments of the LCDIF driver, it's noted:
> >     Set FIFO Panic watermarks, low 1/3, high 2/3 .
> > 
> > However, in the downstream kernels, NXP changes the threshold to
> > 1/2 and 3/4 on the LCDIF that drives the HDMI, while leaving the other
> > LCDIF interfaces at the default.
> > 
> > When I increased the threshold to 1/2 and 3/4, it appeared that several
> > resolutions that my monitor was struggling to sync started working,
> > and it appeared to sync faster.  I don't have an HDMI analyzer, so I
> > cannot verify much beyond knowing if my monitor can or cannot sync.
> 
> You mean the two?
>         thres-low  = <1 2>;             /* (FIFO * 1 / 2) */                                        
>         thres-high = <3 4>;             /* (FIFO * 3 / 4) */
> > 
> > Could the threshold and underrun cause this monitor intermittent issue,
> > or would this be something else?
> 
> The threshold could cause display flicker if the value could not satisfy
> the bandwidth.
> 
> > 
> > Does it make sense to have a flag or device tree option to override the
> > default thresholds to change the panic level?
> 
> I think it would be good to have a property as above.
> 
> + DT maintainer to comment.

Why do you think it's something that should be tuned as a
per-platform/board basis?

If it's something that needs to be adjusted based on the resolution,
then it should always be done in the driver, and that has nothing to do
in the DT.

Maxime
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