linux-kernel - Re: [PATCH v1 0/3] console, serial8250: Disable PM and DMA ops

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Date:   Wed, 18 Jul 2018 18:14:03 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Tony Lindgren <tony@...mide.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v1 0/3] console, serial8250: Disable PM and DMA ops

On Wed, 2018-05-23 at 10:58 -0700, Tony Lindgren wrote:
> * Andy Shevchenko <andy.shevchenko@...il.com> [180522 21:42]:
> > On Thu, May 17, 2018 at 10:30 PM, Tony Lindgren <tony@...mide.com>
> > wrote:
> > > So how about add some "noidle" kernel command line parameter for
> > > console
> > > that calls
> > > pm_runtime_forbid() and then you have the UART permanently
> > > on.
> > 
> > IIUC _forbid() can be overwritten via sysfs.
> > And I would prefer to do other way around, something like
> > console.idle
> > and put default for OMAP to yes and no for everything else.
> 
> OK yeah console.idle sounds good to me. We should default to a
> safe option.

I'll see what we can do here.

> > > Hmm I guess you could make also serial8250_rpm_get() do nothing
> > > based on that.
> > 
> > Have you seen entire series which I keep here:
> > https://bitbucket.org/andy-shev/linux/branch/topic/uart/rpm?
> > Among other things it gets rid of those specific callbacks entirely.
> 
> Well I was not Cc:ed on it, I browsed it in some archive and it
> seemed unsafe to me. But if you figured out a way to do it
> conditionally
> based on console.idle
>  without causing regressions.

I restored that branch with some updated patches. It's far from done and
doesn't have any new stuff (yet) regrading to this discussion.

> > > I do agree the serial runtime PM has an issue if it depends on
> > > pm_runtime_irq_safe() being set.
> > 
> > It's more than an issue. The  so called "support" of RPM for UART is
> > _based on the hack_.
> > I would love to NAK that in the first place if I would have known of
> > it in time.
> 
> Hmm well it seems that you too have been patching the 8250_rpm
> functions for years and then now what after multiple years you
> hit this issue? :)

Nope, I hit it as soon as I tried.

I can't find easily the discussion (hmm... yes, I was so pissed off that
time, I put a bit of harsh in that) I had with Sebastian few years back,
but at least I reported about an issue.

> > > > So, I can, of course just remove callbacks from the console
> > > > ->write().
> > > > Though it will prevent to use kernel console anyway.
> > > 
> > > Please et's not start breaking things, we already see a constant
> > > flow of regressions on weekly basis.
> > 
> > Now we are stick with a hack and the case based on that is against
> > fixing things.
> > This is how it looks from my side.
> 
> Sorry yeah I agree there are issues, but let's fix it properly

Agree.

>  with
> no regressions.

...though I think a word "regression" is inappropriate here. Regression
is what the support did in the first place. Pity I didn't know about it
at that time.

-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy