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

From: Viresh Kumar
Date: Tue Sep 08 2020 - 05:27:44 EST

On 08-09-20, 11:14, Arnd Bergmann wrote:
> Picking up the old thread again after and getting pinged by multiple
> colleagues about it (thanks!) reading through the history.

Thanks for your inputs Arnd.

> 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.
> 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?
> 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. 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 would be fine with both the ideas and that isn't a blocker for me.
Though I still feel this is exactly why we have #mbox-cells here and
that should be enough in this case, even if we opt for multiple

But whatever everyone agrees to will be fine.