Re: [Xen-devel] [GSoC-2019] About the crossbar and xen

[Julien Grall <julien.grall@arm.com>]

Hi,

On 7/11/19 7:29 PM, Hunyue Yau wrote:
> [This mail incorporates comments raised on IRC. I have made some of this mHi,ore
> verbose to provide context to people that haven't seen the IRC comments.]

Thank you for the summary!

> 
> This will be a bunch of facts on the am5. Someone else will have relate it
> back to Xen.
> 
> 1 - The WUGen is a hardware block on the MPU block that can turn interrupts
> into wake up events if the MPU is in certain deeper sleep states. This applies
> only to certain interrupts. We can confirm this by looking at the DT's register
> address. Per the TRM, they are registers for the MPU's PRCM (Power/Reset/Clock
> Management). In short, this block makes interrupts a possible wake up source.
> 
> 2 - Earlier kernels did not expose that HW block. See this patch that
> introduced the WUGen -
> https://github.com/torvalds/linux/commit/7136d457f365ecc93ddffcdd42ab49a8473f260b
> I suspect looking at the before part of the patch should provide clues on how
> to handle the WUGen.
> 
> 
> 3 - This may be redundant but more definitions (in the human sense) here:
> https://www.mjmwired.net/kernel/Documentation/devicetree/bindings/interrupt-controller/ti,omap4-wugen-mpu
> 
> 4 - For the UART case, I suspect the flow Dennis pointed out is about right.
> However, that may be different depending on the interrupt source.
> 
> Unknowns from my point of view -
> 
> a - Does Xen virtualize power management? If so, this may have have an impact.
> I would not recommend adding PM virtualization in GSoC. It is tugging on a
> very long string.

Xen does not virtualize power management at the moment. I agree that 
this is too big for the scope of the GSoC.

> 
> b - If Xen does not virtualize that, someone needs to decide how much to leave
> to the guess.
> 
> c - I wonder if we can do a half way hack where the kernel sets up the PM but
> Xen hooks to get the interrupt. The HW will do its PM thing and Xen can
> process the interrupt.

Hmmm, the question here is whether the UART would be usuable before Dom0 
is setting up the PM? If not, then, we would need to deal with it in Xen.

> 
> Guesses/possible hacks -
> - For the interrupts we care about, the cross bar can route things to the MPU
> unconditionally. This would break the other HW blocks but most of them aren't
> needed for boot.

Sorry for my ignorance, which HW blocks are you talking about?

Cheers,

-- 
Julien Grall

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