[RFC] dt-bindings: mailbox: add doorbell support to ARM MHU

[Sudeep Holla]

On Tue, Sep 08, 2020 at 11:14:50AM +0200, Arnd Bergmann wrote:
> Picking up the old thread again after and getting pinged by multiple
> colleagues about it (thanks!) reading through the history.
> 
> On Fri, Jun 12, 2020 at 7:29 AM Viresh Kumar <viresh.kumar at linaro.org> wrote:
> >
> > On 11-06-20, 19:34, Jassi Brar wrote:
> > > In the first post in this thread, Viresh lamented that mailbox
> > > introduces "a few ms" delay in the scheduler path.
> > > Your own tests show that is certainly not the case -- average is the
> > > same as proposed virtual channels 50-100us, the best case is 3us vs
> > > 53us for virtual channels.
> >
> > Hmmm, I am not sure where is the confusion here Jassi. There are two
> > things which are very very different from each other.
> >
> > - Time taken by the mailbox framework (and remote for acknowledging
> >   it) for completion of a single request, this can be 3us to 100s of
> >   us. This is clear for everyone. THIS IS NOT THE PROBLEM.
> >
> > - Delay introduced by few of such requests on the last one, i.e. 5
> >   normal requests followed by an important one (like DVFS), the last
> >   one needs to wait for the first 5 to finish first. THIS IS THE
> >   PROBLEM.
> 
> Earlier, Jassi also commented "Linux does not provide real-time
> guarantees", which to me is what actually causes the issue here:
> 
> Linux having timeouts when communicating to the firmware means
> that it relies on the hardware and firmware having real-time behavior
> even when not providing real-time guarantees to its processes.
> 
> When comparing the two usage models, it's clear that the minimum
> latency for a message delivery is always at least the time time
> to process an interrupt, plus at least one expensive MMIO read
> and one less expensive posted MMIO write for an Ack. If we
> have a doorbell plus out-of-band message, we need an extra
> DMA barrier and a read from coherent memory, both of which can
> be noticeable. As soon as messages are queued in the current
> model, the maximum latency increases by a potentially unbounded
> number of round-trips, while in the doorbell model that problem
> does not exist, so I agree that we need to handle both modes
> in the kernel deal with all existing hardware as well as firmware
> that requires low-latency communication.
> 
> It also sounds like that debate is already settled because there
> are platforms using both modes, and in the kernel we usually
> end up supporting the platforms that our users have, whether
> we think it's a good idea or not.
>

Thanks for the nice summary of the discussion so far.

> The only questions that I see in need of being answered are:
> 
> 1. Should the binding use just different "#mbox-cells" values or
>    also different "compatible" strings to tell that difference?

I initially proposed latter, but Rob preferred the former which
makes sense for the reasons you have mentioned below.

> 2. Should one driver try to handle both modes or should there
>    be two drivers?
>
> It sounds like Jassi strongly prefers separate drivers, which
> would make separate compatible strings the more practical
> approach.

Indeed.

> While the argument can be made that a single
> piece of hardware should only have one DT description,
> the counter-argument would be that the behavior described
> by the DT here is made up by both the hardware and the
> firmware behind it, and they are in fact different.
>

I am too fine either way.

--
Regards,
Sudeep