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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Nov 27, 2023 at 02:54:21PM +0200, Matti Vaittinen wrote:
> pe 24. marrask. 2023 klo 19.26 Greg Kroah-Hartman
> (gregkh@linuxfoundation.org) kirjoitti:
> >
> > 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 was (am) interested in knowing how/where the regulator error
> notifications are utilized - hence I asked this in ELCE last summer.
> Replies indeed mostly pointed to automotive and handling the under
> voltage events.
> 
> As to what has changed (I think this was asked in another mail on this
> topic) - I understood from the discussions that the demand of running
> systems with as low power as possible is even more
> important/desirable. Hence, the under-voltage events are more usual
> than they were when cars used to be working by burning flammable
> liquids :)
> 
> Anyways, what I thought I'd comment on is that the severity of the
> regulator error notifications can be given from device-tree. Rationale
> behind this is that figuring out whether a certain detected problem is
> fatal or not (in embedded systems) should be done by the board
> designers, per board. Maybe the understanding which hardware should
> react first is also a property of hardware and could come from the
> device-tree? Eg, instead of having a "DEVICE_SHUTDOWN_PRIO_STORAGE"
> set unconditionally for EMMC, systems could set shutdown priority per
> board and per device explicitly using device-tree?

Yes, using device tree would be good, but now you have created something
that is device-tree-specific and not all the world is device tree :(

Also, many devices are finally moving out to non-device-tree busses,
like PCI and USB, so how would you handle them in this type of scheme?

thanks,

greg k-h