linux-kernel - i.MX8MP IMX-LCDIF Underrun Question(s)

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Message-ID: <CAHCN7xLL4hrTK1OqsqUa78cdp9ZcG0sC+cO5QKC3x_Y9-QVzSA@mail.gmail.com>
Date: Thu, 5 Sep 2024 20:46:05 -0500
From: Adam Ford <aford173@...il.com>
To: imx@...ts.linux.dev, Marek Vasut <marex@...x.de>, Stefan Agner <stefan@...er.ch>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Shawn Guo <shawnguo@...nel.org>, Sascha Hauer <s.hauer@...gutronix.de>, 
	Pengutronix Kernel Team <kernel@...gutronix.de>, Fabio Estevam <festevam@...il.com>, 
	dri-devel <dri-devel@...ts.freedesktop.org>, 
	arm-soc <linux-arm-kernel@...ts.infradead.org>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, 
	Schrempf Frieder <frieder.schrempf@...tron.de>, 
	Dominique MARTINET <dominique.martinet@...ark-techno.com>
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.

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