linux-kernel - Re: [PATCH 05/17] clocksource: New RISC-V SBI timer driver

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Message-ID: <24a37ba4-df64-21c4-abec-18c0dbed0ab7@linaro.org>
Date:   Mon, 31 Jul 2017 13:00:26 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Palmer Dabbelt <palmer@...belt.com>, yamada.masahiro@...ionext.com,
        mmarek@...e.com, will.deacon@....com, peterz@...radead.org,
        boqun.feng@...il.com, mingo@...hat.com, tglx@...utronix.de,
        jason@...edaemon.net, marc.zyngier@....com,
        gregkh@...uxfoundation.org, jslaby@...e.com, davem@...emloft.net,
        mchehab@...nel.org, sfr@...b.auug.org.au, fweisbec@...il.com,
        viro@...iv.linux.org.uk, mcgrof@...nel.org, dledford@...hat.com,
        bart.vanassche@...disk.com, sstabellini@...nel.org,
        daniel.vetter@...ll.ch, mpe@...erman.id.au, msalter@...hat.com,
        nicolas.dichtel@...nd.com, james.hogan@...tec.com,
        paul.gortmaker@...driver.com, linux@...ck-us.net,
        heiko.carstens@...ibm.com, schwidefsky@...ibm.com,
        linux-kernel@...r.kernel.org, patches@...ups.riscv.org,
        akpm@...ux-foundation.org
Cc:     albert@...ive.com
Subject: Re: [PATCH 05/17] clocksource: New RISC-V SBI timer driver

On 11/07/2017 03:39, Palmer Dabbelt wrote:
> The RISC-V ISA defines a per-hart real-time clock and timer, which is
> present on all systems.  The clock is accessed via the 'rdtime'
> pseudo-instruction (which reads a CSR), and the timer is set via an SBI
> call.
> 
> This driver attempts to split out the RISC-V ISA specific mechanisms of
> accessing the hardware from the clocksource driver by taking a pair of
> function pointers to issue the actual RISC-V specific instructions.
> 
> Signed-off-by: Palmer Dabbelt <palmer@...belt.com>
> ---

[ ... ]

> +#ifndef _LINUX_TIMER_RISCV_H
> +#define _LINUX_TIMER_RISCV_H
> +
> +/*
> + * All RISC-V systems have a timer attached to every hart.  These timers can be
> + * read by the 'rdcycle' pseudo instruction, and can use the SBI to setup
> + * events.  In order to abstract the architecture-specific timer reading and
> + * setting functions away from the clock event insertion code, we provide
> + * function pointers to the clockevent subsystem that perform two basic operations:
> + * rdtime() reads the timer on the current CPU, and next_event(delta) sets the
> + * next timer event to 'delta' cycles in the future.  As the timers are
> + * inherently a per-cpu resource, these callbacks perform operations on the
> + * current hart.  There is guaranteed to be exactly one timer per hart on all
> + * RISC-V systems.
> + */

Hi Palmer,

The driver code must be self contained (eg. setup and irq handling).

There are examples in the drivers/clocksource to stick on, for example
the jcore-pit.c or the mips-gic-timer.c drivers.

I don't see the point to setup callbacks at init time.

Thanks.

  -- Daniel


> +void timer_riscv_init(int cpu_id,
> +		      unsigned long riscv_timebase,
> +		      unsigned long long (*rdtime)(struct clocksource *),
> +		      int (*next_event)(unsigned long, struct clock_event_device *));
> +
> +/*
> + * Looks up the clocksource or clock_even_device that cooresponds the given
> + * hart.
> + */
> +struct clocksource *timer_riscv_source(int cpuid);
> +struct clock_event_device *timer_riscv_device(int cpu_id);
> +
> +#endif
> 


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