linux-kernel - Re: [RFC PATCH 0/3] Add HiSilicon system timer driver

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Message-ID: <a170493f-cd9f-a0d9-432a-2ae07d18d429@huawei.com>
Date:   Wed, 11 Oct 2023 10:10:11 +0800
From:   Yicong Yang <yangyicong@...wei.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     <yangyicong@...ilicon.com>, <mark.rutland@....com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <daniel.lezcano@...aro.org>,
        <tglx@...utronix.de>, <jonathan.cameron@...wei.com>,
        <prime.zeng@...wei.com>, <wanghuiqiang@...wei.com>,
        <wangwudi@...ilicon.com>, <guohanjun@...wei.com>,
        <linuxarm@...wei.com>
Subject: Re: [RFC PATCH 0/3] Add HiSilicon system timer driver

On 2023/10/11 0:36, Marc Zyngier wrote:
> On Tue, 10 Oct 2023 13:30:30 +0100,
> Yicong Yang <yangyicong@...wei.com> wrote:
>>
>> From: Yicong Yang <yangyicong@...ilicon.com>
>>
>> HiSilicon system timer is a memory mapped platform timer compatible with
>> the arm's generic timer specification. The timer supports both SPI and
>> LPI interrupt and can be enumerated through ACPI DSDT table. Since the
>> timer is fully compatible with the spec, it can reuse most codes of the
>> arm_arch_timer driver. However since the arm_arch_timer driver only
>> supports GTDT and SPI interrupt, this series support the HiSilicon system
>> timer by:
>>
>> - refactor some of the arm_arch_timer codes and export the function to
>>   register a arch memory timer by other drivers
>> - retrieve the IO memory and interrupt resource through DSDT in a separate
>>   driver, then setup and register the clockevent device reuse the arm_arch_timer
>>   function
>>
>> Using LPI for the timer is mentioned in BSA Spec section 3.8.1 (DEN0094C 1.0C).
> 
> This strikes me as pretty odd. LPIs are, by definition, *edge*
> triggered. The timer interrupt must be *level* triggered. So there
> must be some bridge in the middle that is going to regenerate edges on
> EOI, and that cannot be architectural.
> 
> What am I missing?

In our case, if the timer is working on LPI mode, it's not directly connected
to the GIC. It'll be wired to hisi-mbigen irqchip which will send LPIs to the
GIC.

Thanks.