Re: [EXTERNAL][RFC PATCH v3 07/12] genirq: Shutdown irq chips in suspend/resume during hibernation

[Anchal Agarwal]

On Sat, Mar 07, 2020 at 12:03:52AM +0100, Thomas Gleixner wrote:
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> 
> 
> Anchal Agarwal <anchalag@xxxxxxxxxx> writes:
> 
> > There are no pm handlers for the legacy devices, so during tear down
> > stale event channel <> IRQ mapping may still remain in the image and
> > resume may fail. To avoid adding much code by implementing handlers for
> > legacy devices, add a new irq_chip flag IRQCHIP_SHUTDOWN_ON_SUSPEND which
> > when enabled on an irq-chip e.g xen-pirq, it will let core suspend/resume
> > irq code to shutdown and restart the active irqs. PM suspend/hibernation
> > code will rely on this.
> > Without this, in PM hibernation, information about the event channel
> > remains in hibernation image, but there is no guarantee that the same
> > event channel numbers are assigned to the devices when restoring the
> > system. This may cause conflict like the following and prevent some
> > devices from being restored correctly.
> 
> The above is just an agglomeration of words and acronyms and some of
> these sentences do not even make sense. Anyone who is not aware of event
> channels and whatever XENisms you talk about will be entirely
> confused. Changelogs really need to be understandable for mere mortals
> and there is no space restriction so acronyms can be written out.
> 
I don't understand what does not makes sense here. Of course the one you
described is more elaborate and explanatory and I agree I just wrote a short 
one from perspective of PM hibernation related to Xen domU. 
All I explained was why teardown is needed, what is the solution and 
what will happen if we do not clear those mappings. 
> Something like this:
> 
>   Many legacy device drivers do not implement power management (PM)
>   functions which means that interrupts requested by these drivers stay
>   in active state when the kernel is hibernated.
> 
>   This does not matter on bare metal and on most hypervisors because the
>   interrupt is restored on resume without any noticable side effects as
>   it stays connected to the same physical or virtual interrupt line.
> 
>   The XEN interrupt mechanism is different as it maintains a mapping
>   between the Linux interrupt number and a XEN event channel. If the
>   interrupt stays active on hibernation this mapping is preserved but
>   there is unfortunately no guarantee that on resume the same event
>   channels are reassigned to these devices. This can result in event
>   channel conflicts which prevent the affected devices from being
>   restored correctly.
> 
>   One way to solve this would be to add the necessary power management
>   functions to all affected legacy device drivers, but that's a
>   questionable effort which does not provide any benefits on non-XEN
>   environments.
> 
>   The least intrusive and most efficient solution is to provide a
>   mechanism which allows the core interrupt code to tear down these
>   interrupts on hibernation and bring them back up again on resume. This
>   allows the XEN event channel mechanism to assign an arbitrary event
>   channel on resume without affecting the functionality of these
>   devices.
> 
>   Fortunately all these device interrupts are handled by a dedicated XEN
>   interrupt chip so the chip can be marked that all interrupts connected
>   to it are handled this way. This is pretty much in line with the other
>   interrupt chip specific quirks, e.g. IRQCHIP_MASK_ON_SUSPEND.
> 
>   Add a new quirk flag IRQCHIP_SHUTDOWN_ON_SUSPEND and add support for
>   it the core interrupt suspend/resume paths.
> 
> Hmm?
> 
Sure.
> > Signed-off-by: Anchal Agarwal <anchalag@xxxxxxxxxx>
> > Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> Not that I care much, but now that I've written both the patch and the
> changelog you might change that attribution slightly. For completeness
> sake:
> 
Why not. That's mandated now :)
>  Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> Thanks,
> 
>         tglx
Thanks,
Anchal