i.MX8MP IMX-LCDIF Underrun Question(s)

[Peng Fan]

> Subject: Re: i.MX8MP IMX-LCDIF Underrun Question(s)
> 
> 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.
> 

Downstream kernel has some NOC settings that not supported
in upstream kernel, that maybe the issue.

If you check 6.6 kernel, you could see some noc related settings
in imx8mp-blk-ctl.c imx8m-blk-ctl.c and gpcv2.c. You may give a
try with those noc settings applied and see whether that would
improve.

Regards,
Peng.

> >
> > 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