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

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Message-ID: <01d578b9-e42e-4767-a33f-b0892a602e23@kontron.de>
Date: Fri, 6 Sep 2024 22:01:34 +0200
From: Frieder Schrempf <frieder.schrempf@...tron.de>
To: Adam Ford <aford173@...il.com>, 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>,
 Dominique MARTINET <dominique.martinet@...ark-techno.com>
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.

> 
> 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