linux-kernel - Re: [PATCH v1 0/3] introduce priority-based shutdown support

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Message-ID: <2023112453-flagstick-bullring-8511@gregkh>
Date:   Fri, 24 Nov 2023 17:26:30 +0000
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
Cc:     Mark Brown <broonie@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>, kernel@...gutronix.de,
        linux-kernel@...r.kernel.org, linux-mmc@...r.kernel.org,
        linux-pm@...r.kernel.org,
        Søren Andersen <san@...v.dk>
Subject: Re: [PATCH v1 0/3] introduce priority-based shutdown support

On Fri, Nov 24, 2023 at 05:32:34PM +0100, Oleksij Rempel wrote:
> On Fri, Nov 24, 2023 at 03:56:19PM +0000, Greg Kroah-Hartman wrote:
> > On Fri, Nov 24, 2023 at 03:49:46PM +0000, Mark Brown wrote:
> > > On Fri, Nov 24, 2023 at 03:27:48PM +0000, Greg Kroah-Hartman wrote:
> > > > On Fri, Nov 24, 2023 at 03:21:40PM +0000, Mark Brown wrote:
> > > 
> > > > > This came out of some discussions about trying to handle emergency power
> > > > > failure notifications.
> > > 
> > > > I'm sorry, but I don't know what that means.  Are you saying that the
> > > > kernel is now going to try to provide a hard guarantee that some devices
> > > > are going to be shut down in X number of seconds when asked?  If so, why
> > > > not do this in userspace?
> > > 
> > > No, it was initially (or when I initially saw it anyway) handling of
> > > notifications from regulators that they're in trouble and we have some
> > > small amount of time to do anything we might want to do about it before
> > > we expire.
> > 
> > So we are going to guarantee a "time" in which we are going to do
> > something?  Again, if that's required, why not do it in userspace using
> > a RT kernel?
> 
> For the HW in question I have only 100ms time before power loss. By
> doing it over use space some we will have even less time to react.

Why can't userspace react that fast?  Why will the kernel be somehow
faster?  Speed should be the same, just get the "power is cut" signal
and have userspace flush and unmount the disk before power is gone.  Why
can the kernel do this any differently?

> In fact, this is not a new requirement. It exist on different flavors of
> automotive Linux for about 10 years. Linux in cars should be able to
> handle voltage drops for example on ignition and so on. The only new thing is
> the attempt to mainline it.

But your patch is not guaranteeing anything, it's just doing a "I want
this done before the other devices are handled", that's it.  There is no
chance that 100ms is going to be a requirement, or that some other
device type is not going to come along and demand to be ahead of your
device in the list.

So you are going to have a constant fight among device types over the
years, and people complaining that the kernel is now somehow going to
guarantee that a device is shutdown in a set amount of time, which
again, the kernel can not guarantee here.

This might work as a one-off for a specific hardware platform, which is
odd, but not anything you really should be adding for anyone else to use
here as your reasoning for it does not reflect what the code does.

thanks,

greg k-h