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. Right. > 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 drivers. But whatever everyone agrees to will be fine. -- viresh
