linux-kernel - Re: [PATCH 09/15] irq: arm: perform irqentry in entry code

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Date:   Sat, 23 Oct 2021 14:36:52 +0100
From:   Vladimir Murzin <vladimir.murzin@....com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
        aou@...s.berkeley.edu, catalin.marinas@....com,
        deanbo422@...il.com, green.hu@...il.com, guoren@...nel.org,
        jonas@...thpole.se, kernelfans@...il.com,
        linux-arm-kernel@...ts.infradead.org, linux@...linux.org.uk,
        nickhu@...estech.com, palmer@...belt.com, paulmck@...nel.org,
        paul.walmsley@...ive.com, peterz@...radead.org, shorne@...il.com,
        stefan.kristiansson@...nalahti.fi, tglx@...utronix.de,
        torvalds@...ux-foundation.org, tsbogend@...ha.franken.de,
        vgupta@...nel.org, will@...nel.org
Subject: Re: [PATCH 09/15] irq: arm: perform irqentry in entry code

On 10/23/21 2:18 PM, Marc Zyngier wrote:
> On Sat, 23 Oct 2021 13:06:25 +0100,
> Vladimir Murzin <vladimir.murzin@....com> wrote:
>>
>> On 10/22/21 7:43 PM, Marc Zyngier wrote:
>>> On Fri, 22 Oct 2021 18:58:54 +0100,
>>> Mark Rutland <mark.rutland@....com> wrote:
>>>>
>>>> On Fri, Oct 22, 2021 at 05:34:20PM +0100, Vladimir Murzin wrote:
> 
> [...]
> 
>>>>> As for TODO, is [1] look something you have been thinking of? IIUC,
>>>>> the show stopper is that hwirq is being passed from exception entry
>>>>> which retrieved via xPSR (IPSR to be precise). OTOH hwirq also available
>>>>> via Interrupt Controller Status Register (ICSR) thus can be used in
>>>>> driver itself... I gave [1] a go and it runs fine, yet I admit I might
>>>>> be missing something...
>>>>
>>>> I hadn't thought about it in much detail, but that looks good!
>>>>
>>>> I was wondering if we needed something like a
>>>> handle_arch_vectored_irq(), but if we can rely on the ICSR that seems
>>>> simpler overall. I'm not at all familiar with M-class, so I'm not sure
>>>> if there are pitfalls in this area.
>>>
>>> Why can't we just use IPSR instead from the C code? It has the
>>> potential of being of lower latency then a MMIO read (though I have no
>>> idea whether it makes a material difference on M-class) and from what
>>> I can see in the arch spec, they are strictly equivalent.
>>
>> Hmmm, less arch specific asm(s) in driver code, no?
> 
> Well, it isn't like this driver is going to be useful on anything
> else, is it?
> 

Well, with some work to unwire it from arch/arm it can be COMPILE_TEST :)

> If there is no overhead in reading from MMIO compared to the
> architected register, then I agree that ICSR is the way to
> go. Is there any chance you could measure it on a HW platform? Or
> maybe in emulation?

My MPS{2,3} boards left in office and I'm on holiday next week... OTOH, I
have no strong opinion on ICSR vs IPSR, I just wanted to check how much
work it'd be to close TODO per my (quite limited) understanding :)

Cheers
Vladimir


> 
> Thanks,
> 
> 	M.
>