Re: [PATCH v2 01/11] clocksource: timer-riscv: select CSR from device tree

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Antony,

On 07.05.21 00:08, Antony Pavlov wrote:
> barebox timer-riscv driver supports one of user counters:
> 
>   * 'cycle', counter for RDCYCLE instruction (CSR 0xc00);
>   * 'time', timer for RDTIME instruction (CSR 0xc01).
> 
> At the moment in S-mode timer-riscv uses the 'cycle' counter,
> and in M-mode timer-riscv uses the 'time' timer.

Other way round, right? cycle for M-Mode, time for S-Mode?

> 
> Alas picorv32 CPU core supports only the 'cycle' counter.
> VexRiscV CPU core supports only the 'time' timer.
> 
> This patch makes it possible to use the 'time' timer
> for VexRiscV CPU in M-mode.

Is that allowed by the ISA? To provide time, but not cycle?
Can VexRiscV boot Linux? If so, how does Linux handle lack
of this CSR?
 
> Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
> ---
>  arch/riscv/cpu/time.c             |  7 +++++++
>  arch/riscv/dts/erizo.dtsi         |  2 ++
>  arch/riscv/include/asm/timer.h    |  1 +
>  drivers/clocksource/timer-riscv.c | 19 ++++++++-----------
>  4 files changed, 18 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/riscv/cpu/time.c b/arch/riscv/cpu/time.c
> index 39bb6a5112..59c8ca61d6 100644
> --- a/arch/riscv/cpu/time.c
> +++ b/arch/riscv/cpu/time.c
> @@ -18,6 +18,7 @@
>  #include <asm/timer.h>
>  
>  unsigned long riscv_timebase;
> +unsigned long riscv_use_csr_cycle;
>  
>  int timer_init(void)
>  {
> @@ -32,6 +33,12 @@ int timer_init(void)
>  
>  	riscv_timebase = prop;
>  
> +	if (of_property_read_bool(cpu, "csr-cycle")) {
> +		riscv_use_csr_cycle = 1;
> +	} else {
> +		riscv_use_csr_cycle = 0;
> +	}
> +

Any reason this couldn't happen in driver probe?
I'd also prefer another name, e.g. barebox,use-csr-cycle ?

>  	of_platform_populate(cpu, NULL, NULL);
>  
>  	return 0;
> diff --git a/arch/riscv/dts/erizo.dtsi b/arch/riscv/dts/erizo.dtsi
> index 228711bd69..b3ccf281f2 100644
> --- a/arch/riscv/dts/erizo.dtsi
> +++ b/arch/riscv/dts/erizo.dtsi
> @@ -22,6 +22,8 @@
>  
>  		timebase-frequency = <24000000>;
>  
> +		csr-cycle;
> +
>  		cpu@0 {
>  			device_type = "cpu";
>  			compatible = "cliffordwolf,picorv32", "riscv";
> diff --git a/arch/riscv/include/asm/timer.h b/arch/riscv/include/asm/timer.h
> index 1f78ef4c00..555b3f5989 100644
> --- a/arch/riscv/include/asm/timer.h
> +++ b/arch/riscv/include/asm/timer.h
> @@ -5,5 +5,6 @@
>  
>  int timer_init(void);
>  extern unsigned long riscv_timebase;
> +extern unsigned long riscv_use_csr_cycle;
>  
>  #endif /* _ASM_RISCV_DELAY_H */
> diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
> index ef67cff475..c0deed40eb 100644
> --- a/drivers/clocksource/timer-riscv.c
> +++ b/drivers/clocksource/timer-riscv.c
> @@ -13,7 +13,7 @@
>  #include <asm/timer.h>
>  #include <asm/csr.h>
>  
> -static u64 notrace riscv_timer_get_count_sbi(void)
> +static u64 notrace riscv_timer_get_count_time(void)
>  {
>  	__maybe_unused u32 hi, lo;
>  
> @@ -28,7 +28,7 @@ static u64 notrace riscv_timer_get_count_sbi(void)
>  	return ((u64)hi << 32) | lo;
>  }
>  
> -static u64 notrace riscv_timer_get_count_rdcycle(void)
> +static u64 notrace riscv_timer_get_count_cycle(void)
>  {
>  	__maybe_unused u32 hi, lo;
>  
> @@ -43,16 +43,7 @@ static u64 notrace riscv_timer_get_count_rdcycle(void)
>  	return ((u64)hi << 32) | lo;
>  }
>  
> -static u64 notrace riscv_timer_get_count(void)
> -{
> -	if (IS_ENABLED(CONFIG_RISCV_SBI))
> -		return riscv_timer_get_count_sbi();
> -	else
> -		return riscv_timer_get_count_rdcycle();
> -}
> -
>  static struct clocksource riscv_clocksource = {
> -	.read		= riscv_timer_get_count,
>  	.mask		= CLOCKSOURCE_MASK(64),
>  	.priority	= 100,
>  };
> @@ -61,6 +52,12 @@ static int riscv_timer_init(struct device_d* dev)
>  {
>  	dev_info(dev, "running at %lu Hz\n", riscv_timebase);
>  
> +	if (riscv_use_csr_cycle) {
> +		riscv_clocksource.read = riscv_timer_get_count_cycle;
> +	} else {
> +		riscv_clocksource.read = riscv_timer_get_count_time;
> +	}
> +
>  	riscv_clocksource.mult = clocksource_hz2mult(riscv_timebase, riscv_clocksource.shift);
>  
>  	return init_clock(&riscv_clocksource);
> 

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