linux-kernel - Re: [PATCH] serial: 8250_dw: Fix runtime PM handling

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Date:   Mon, 1 Oct 2018 10:23:57 -0700
From:   Tony Lindgren <tony@...mide.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Phil Edworthy <phil.edworthy@...esas.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH] serial: 8250_dw: Fix runtime PM handling

* Guenter Roeck <linux@...ck-us.net> [180928 22:45]:
> On Thu, May 10, 2018 at 03:13:56PM +0100, Phil Edworthy wrote:
> > When using kgdb, you get an abort when accessing the UART registers.
> > This is because the driver has already entered runtime PM and so turned
> > off the bus clock needed to access the registers.
> > 
> > To fix this, set the capability indicating Runtime PM is active while idle.
> > 
> 
> With this patch applied, I get the already reported traceback [1].
> Obviously neither the promised revert [2] nor the presumed fix [3] made it
> into the mainline Kernel.
> 
> Greg, can you please revert this patch ? If people don't care enough
> to fix the problem, we should at least not have to suffer the consequences.

Yes please revert until things are fixed.

FYI, what Andy and I discussed earlier is that we can disable
runtime PM for serial layer by taking a PM runtime use count
on the serial driver if there is an active console but we need
a way to attach/detach kernel serial consoel via /sys to prevent
regressions with PM.

So how do we attach/detach kernel serial console? We don't seem
to have a way to attach/detach a kernel console to a UART via
/sys do we? Or am I again 10 years behind or something?

We have /sys/class/vtconsole to bind and unbind a frame
buffer console though.

Andy, do you have your updated patches available somewhere?

Regards,

Tony

> [1] https://www.spinics.net/lists/linux-serial/msg30379.html
> [2] https://www.spinics.net/lists/linux-serial/msg30380.html
> [3] https://www.spinics.net/lists/linux-serial/msg30757.html