linux-kernel - Re: [PATCH] cpuidle: riscv-sbi: Stop using non-retentive suspend

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Date:   Tue, 22 Nov 2022 23:38:22 -0600
From:   Samuel Holland <samuel@...lland.org>
To:     Anup Patel <apatel@...tanamicro.com>
Cc:     Palmer Dabbelt <palmer@...osinc.com>, conor.dooley@...rochip.com,
        anup@...infault.org, Conor Dooley <conor@...nel.org>,
        rafael@...nel.org, daniel.lezcano@...aro.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, linux-pm@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux@...osinc.com
Subject: Re: [PATCH] cpuidle: riscv-sbi: Stop using non-retentive suspend

Hi Anup,

On 11/22/22 23:35, Anup Patel wrote:
> On Wed, Nov 23, 2022 at 10:41 AM Samuel Holland <samuel@...lland.org> wrote:
>> On 11/22/22 09:28, Palmer Dabbelt wrote:
>>> I also think we should stop entering non-retentive suspend until we can
>>> sort out how reliably wake up from it, as the SBI makes that a
>>> platform-specific detail.  If the answer there is "non-retentive suspend
>>> is fine on the D1 as long as we don't use the SBI timers" then that
>>> seems fine, we just need some way to describe that in Linux -- that
>>> doesn't fix other platforms and other interrupts, but at least it's a
>>> start.
>>
>> We need some way to describe the situation from the SBI implementation
>> to Linux.
>>
>> Non-retentive suspend is fine on the D1 as long as either one of these
>> conditions is met:
>>  1) we don't use the SBI timers, or
>>  2) the SBI timer implementation does not use the CLINT
>>
>> And it is up to the SBI implementation which timer hardware it uses, so
>> the SBI implementation needs to patch this information in to the DT at
>> runtime.
> 
> Rather than SBI implementation patching information in DT, it is much
> simpler to add a quirk in RISC-V timer driver for D1 platform (i.e. based
> on D1 compatible string in root node).

It would be simpler, but it would be wrong, as I just explained.

Only the SBI implementation knows if the SBI timer extension can wake
any given CPU from any given non-retentive suspend state.

Regards,
Samuel