i.MX8MP IMX-LCDIF Underrun Question(s)

[Frieder Schrempf]

On 06.09.24 3:46 AM, Adam Ford wrote:
> 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.

For me this change doesn't seem to cause any improved behavior. My
monitor still fails to sync every few times I run "modetest -s" .

Also we have a downstream kernel based on 6.1 with backported HDMI
support and I don't see the issues there. But I need to make some
further tests to make any reliable statements.

> 
> Could the threshold and underrun cause this monitor intermittent
> issue, or would this be something else?
> 
> Does it make sense to have a flag or device tree option to override
> the default thresholds to change the panic level?
> 
> adam