i.MX8MP IMX-LCDIF Underrun Question(s)

[Peng Fan]

> 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