linux-kernel - Re: [RFC][PATCH] serial: amba-pl011: Make sure we initialize the port.lock spinlock

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Date:   Fri, 24 Apr 2020 21:04:50 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org
Subject: Re: [RFC][PATCH] serial: amba-pl011: Make sure we initialize the
 port.lock spinlock

On Thu, Apr 23, 2020 at 4:14 PM Valentin Schneider
<valentin.schneider@....com> wrote:
> On 23/04/20 23:00, John Stultz wrote:
> > Which seems to be due to the fact that after allocating the uap
> > structure, the pl011 code doesn't initialize the spinlock.
> >
> > This patch fixes it by initializing the spinlock and the warning
> > has gone away.
> >
>
> Thanks for having a look. It does seem like the reasonable thing to do, and
> I no longer get the warning on h960.
>
> That said, I got more curious as this doesn't show up on my Juno (same
> Image). Digging into it I see that uart_add_one_port() has a call to
> uart_port_spin_lock_init() a few lines before uart_configure_port() (in
> which the above warning gets triggered). That thing says:
>
>  * Ensure that the serial console lock is initialised early.
>  * If this port is a console, then the spinlock is already initialised.
>
> Which requires me to ask: are we doing the right thing here?

So I got a little bit of time to look at this before I got pulled off
to other things (and now its Friday night, so I figured I'd reply
before I forget it on Monday).

I did check and lockdep is tripping when we add ttyAMA6 which is the
serial console on the board. I wasn't able to trace back to why we
hadn't already called spin_lock_init() in the console code, but it
seems we haven't.

Also I checked on HiKey as well, and there I'm seeing the same lockdep
splat and this fix seems to resolve it. So more digging is needed. If
anyone has a better idea of what might be awry or if the lock does
need to be initialized in the driver (it's a bit inconsistent, I see
some drivers do but others don't), let me know.

thanks
-john