linux-kernel - High interrupt latency with low power idle mode on i.MX6

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Message-ID: <bc91129c-121c-a070-53b2-1f0bb6d4500a@kontron.de>
Date:   Wed, 27 May 2020 10:39:12 +0000
From:   Schrempf Frieder <frieder.schrempf@...tron.de>
To:     Russell King <linux@...linux.org.uk>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Enrico Weigelt <info@...ux.net>,
        "Thomas Gleixner" <tglx@...utronix.de>
CC:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: High interrupt latency with low power idle mode on i.MX6

Hi,

on our i.MX6UL/ULL boards running mainline kernels, we see an issue with 
RS485 collisions on the bus. These are caused by the resetting of the 
RTS signal being delayed after each transmission. The TXDC interrupt 
takes several milliseconds to trigger and the slave on the bus already 
starts to send a reply in the meantime.

We found out that these delays only happen when the CPU is in "low power 
idle" mode (ARM power off). When we disable cpuidle state 2 or put some 
background load on the CPU everything works fine and the delays are gone.

echo 1 > /sys/devices/system/cpu/cpu0/cpuidle/state2/disable

It seems like also other interfaces (I2C, etc.) might be affected by 
these increased latencies, we haven't investigated this more closely, 
though.

We currently apply a patch to our kernel, that disables low power idle 
mode by default, but I'm wondering if there's a way to fix this 
properly? Any ideas?

Thanks,
Frieder