linux-kernel - Re: [PATCH 1/3 v2] clocksource/drivers/fttmr010: Be stricter on IRQs

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Message-ID: <0c180f94-50df-1cbe-3cf6-62904b017a8c@kaod.org>
Date:   Fri, 24 Sep 2021 15:56:36 +0200
From:   Cédric Le Goater <clg@...d.org>
To:     Linus Walleij <linus.walleij@...aro.org>
CC:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Joel Stanley <joel@....id.au>,
        Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH 1/3 v2] clocksource/drivers/fttmr010: Be stricter on IRQs

On 9/23/21 23:05, Linus Walleij wrote:
> On Thu, Sep 23, 2021 at 10:11 PM Cédric Le Goater <clg@...d.org> wrote:
> 
>> I think we should start by dropping all the AST2600 code which
>> is unused.
> 
> I don't see why, the hardware is there is it not?

The TMC34 register is different on the AST2600.

The only piece of code that makes sense is in ast2600_timer_interrupt() :
     
	writel(0x1, fttmr010->base + TIMER_INTR_STATE);

which clears the status.

If you really insist on keeping the AST2600 support, then I would
rework a bit ast2600_timer_interrupt() : drop TIMER_1_INT_OVERFLOW
and may be use BIT(0) instead of TIMER_1_INT_MATCH1, since the
register layout is different.

There are 8 timers on the AST2600.

Thanks,

C.

> In my experience it is unwise to try to system manage the kernel,
> decide what hardware gets exposed to the frameworks and which
> does not.
> 
> There have been instances in the past where we have first said we don't
> need another timer on the system (so it is "dark silicon") and later brought
> it back because it has some upside.
> 
> For example for a while the Ux500 was using clksrc-dbx500-prcmu.c
> exclusively because it was the only clocksource that would not stop
> during sleep, and nomadik-mtu.c was unused. Then we invented a
> way to grade the different clocksources and switch between them
> before sleep, but tagging one of them with
> CLOCK_SOURCE_SUSPEND_NONSTOP and giving them the right
> rating, see commit bc0750e464d4.
> 
> This was good because nomadi-mtu.c has higher granularity and
> higher frequency when the system is awake but clksrc-dbx500-prcmu.c
> is always ticking, so each is used for different purposes.
> 
> Lesson learned: register all hardware with the timekeeping core and
> let the kernel decide what timer to use at what point and for what.
> 
> Yours,
> Linus Walleij
>