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

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Message-ID: <20231124185725.GA872366@pengutronix.de>
Date:   Fri, 24 Nov 2023 19:57:25 +0100
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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 05:26:30PM +0000, Greg Kroah-Hartman wrote:
> 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.

I see. Good point.

In my case umount is not needed, there is not enough time to write down
the data. We should send a shutdown command to the eMMC ASAP.

@Ulf, are there a way request mmc shutdown from user space?
If I see it correctly, sysfs-devices-power-control support only "auto" and
"on". Unbinding the module will not execute MMC shutdown notification.
If user space is the way to go, do sysfs-devices-power-control "off"
command will be acceptable?

The other option I have is to add a regulator event handler to the MMC
framework and do shutdown notification on under-voltage event.

Are there other options?

Regards,
Oleksij
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