linux-kernel - Re: [PATCH] serial: core: Fix checks for tx runtime PM state

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Message-ID: <20231018050727.GI34982@atomide.com>
Date:   Wed, 18 Oct 2023 08:07:27 +0300
From:   Tony Lindgren <tony@...mide.com>
To:     Johan Hovold <johan@...nel.org>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>, Dhruva Gole <d-gole@...com>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        John Ogness <john.ogness@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
        Maximilian Luz <luzmaximilian@...il.com>
Subject: Re: [PATCH] serial: core: Fix checks for tx runtime PM state

* Johan Hovold <johan@...nel.org> [231016 15:18]:
> On Sat, Oct 07, 2023 at 08:45:41AM +0300, Tony Lindgren wrote:
> > * Johan Hovold <johan@...nel.org> [231006 15:37]:
> > > On Fri, Oct 06, 2023 at 11:37:12AM +0300, Tony Lindgren wrote:
> 
> > > > Care to clarify a bit which parts are unclear? The hierarchy of port
> > > > devices, making serial core manage runtime PM in a generic way, or
> > > > flushing tx?
> > > 
> > > I still don't know why you added these two new abstractions (controller
> > > and port), and that isn't really explained by the commit message either.
> > 
> > We want serial core to do runtime PM in a generic way and have the usage
> > count propagate to the parent serial port hardware device. This way we
> > don't need to care much if the numerous serial port drivers implement
> > runtime PM or not. Well, except for now we need to check the parent state
> > for this fix :)
> 
> That sounds like a lot of complexity to avoid checking if (the single
> instance of) pm_runtime_get() returns -EACCESS.

Yes only one call so far. but we have the serial core generic PM patch(es)
from Andy and Ilpo that are still coming.

> > We also want serial core to know the serial port to serial port hardware
> > mapping as we already have multiport devices. The serial core controller
> > is there to group the serial ports for each serial port hardware device.
> > We at least now have an option to support devices with multiple controllers
> > and ports in case we ever happen to see such things.
> 
> Hypothetical multiple serial controllers should be modelled as separate
> controllers, but yeah, perhaps we want to describe the ports.

Yes and we already have multiport controllers.

> > > And if these are indeed needed, then why isn't the serdev controller now
> > > a child of the "port" device, for example?
> > 
> > Yes I agree we should now move serdev controller to be a child of the
> > serial core port device. Then this $subject patch can be reverted.
> > 
> > Moving serdev controller should also help serdev to deal with multiport
> > devices I think?
> 
> It wouldn't help currently I think, since we already resume the
> controller and don't manage ports individually, but if we now have port
> devices then it probably should be moved.

I'll post a patch for that after some more checks.

Regards,

Tony