RE: i.MX8MP IMX-LCDIF Underrun Question(s)

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.

Regards
Peng.

adam