From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
To: Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
Tony Lindgren <tony@atomide.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Jiri Slaby <jslaby@suse.com>,
linux-serial@vger.kernel.org,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: [PATCH v2 5/8] serial: 8250_port: Don't use power management for kernel console
Date: Fri, 14 Feb 2020 13:39:18 +0000 [thread overview]
Message-ID: <20200214133918.GP25745@shell.armlinux.org.uk> (raw)
In-Reply-To: <20200214114339.53897-6-andriy.shevchenko@linux.intel.com>
On Fri, Feb 14, 2020 at 01:43:36PM +0200, Andy Shevchenko wrote:
> Doing any kind of power management for kernel console is really bad idea.
>
> First of all, it runs in poll and atomic mode. This fact attaches a limitation
> on the functions that might be called. For example, pm_runtime_get_sync() might
> sleep and thus can't be used. This call needs, for example, to bring the device
> to powered on state on the system, where the power on sequence may require
> on-atomic operations, such as Intel Cherrytrail with ACPI enumerated UARTs.
> That said, on ACPI enabled platforms it might even call firmware for a job.
>
> On the other hand pm_runtime_get() doesn't guarantee that device will become
> powered on fast enough.
>
> Besides that, imagine the case when console is about to print a kernel Oops and
> it's powered off. In such an emergency case calling the complex functions is
> not the best what we can do, taking into consideration that user wants to see
> at least something of the last kernel word before it passes away.
>
> Here we modify the 8250 console code to prevent runtime power management.
It's probably also worth noting (and documenting) that this will likely
cause a PM regression for OMAP platforms since the serial port will no
longer be idled, and therefore the power domains will not hit retention
state. Please wait for Tony to confirm.
--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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next prev parent reply other threads:[~2020-02-14 13:39 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-14 11:43 [PATCH v2 0/8] serial: Disable DMA and PM on kernel console Andy Shevchenko
2020-02-14 11:43 ` [PATCH v2 1/8] serial: core: Introduce uart_console_enabled() helper Andy Shevchenko
2020-02-14 11:43 ` [PATCH v2 2/8] serial: core: Consolidate spin lock initialization code Andy Shevchenko
2020-02-14 11:43 ` [PATCH v2 3/8] serial: core: use octal permissions on module param Andy Shevchenko
2020-02-14 16:27 ` Greg Kroah-Hartman
2020-02-17 9:15 ` Andy Shevchenko
2020-02-14 11:43 ` [PATCH v2 4/8] serial: core: Allow detach and attach serial device for console Andy Shevchenko
2020-02-14 16:28 ` Greg Kroah-Hartman
2020-02-17 10:23 ` Andy Shevchenko
2020-02-14 11:43 ` [PATCH v2 5/8] serial: 8250_port: Don't use power management for kernel console Andy Shevchenko
2020-02-14 13:39 ` Russell King - ARM Linux admin [this message]
2020-02-14 17:13 ` Tony Lindgren
2020-02-14 18:09 ` Andy Shevchenko
2020-02-14 18:42 ` Tony Lindgren
2020-02-14 19:06 ` Tony Lindgren
2020-02-14 18:14 ` Tony Lindgren
2020-02-14 18:56 ` Andy Shevchenko
2020-02-14 19:02 ` Tony Lindgren
2020-02-14 11:43 ` [PATCH v2 6/8] serial: 8250_port: Disable DMA operations " Andy Shevchenko
2020-02-14 16:30 ` Greg Kroah-Hartman
2020-02-17 9:18 ` Andy Shevchenko
2020-02-14 11:43 ` [PATCH v2 7/8] serial: 8250_mtk: Remove duplicating code to disable DMA Andy Shevchenko
2020-02-14 11:43 ` [PATCH v2 8/8] serial: 8250_omap: " Andy Shevchenko
2020-02-14 16:33 ` [PATCH v2 0/8] serial: Disable DMA and PM on kernel console Greg Kroah-Hartman
2020-02-14 21:41 ` Tony Lindgren
2020-02-17 10:05 ` Andy Shevchenko
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