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

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Message-ID:
 <PAXPR04MB84592996FD8101BC0CF9E366889E2@PAXPR04MB8459.eurprd04.prod.outlook.com>
Date: Fri, 6 Sep 2024 08:03:38 +0000
From: Peng Fan <peng.fan@....com>
To: Adam Ford <aford173@...il.com>, "imx@...ts.linux.dev"
	<imx@...ts.linux.dev>, Marek Vasut <marex@...x.de>, Stefan Agner
	<stefan@...er.ch>, Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
	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>, "robh@...nel.org" <robh@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>
Subject: RE: i.MX8MP IMX-LCDIF Underrun Question(s)

> 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