[PATCH] RISC-V: Resurrect the MMIO timer implementation for M-mode systems

[Damien Le Moal]

On Mon, 2020-09-14 at 09:56 -0700, Palmer Dabbelt wrote:
> The K210 doesn't implement rdtime in M-mode, and since that's where Linux runs
> in the NOMMU systems that means we can't use rdtime.  The K210 is the only
> system that anyone is currently running NOMMU or M-mode on, so here we're just
> inlining the timer read directly.
> 
> Signed-off-by: Palmer Dabbelt <palmerdabbelt at google.com>
> ---
> I don't actually have a K210 so I haven't tested this.  If nobody else has the
> time to I'll put together a QEMU that doesn't support rdtime in M-mode, but
> I've yet to mess around with the !MMU stuff so that might take a little while.
> This certainly doesn't seem worse than what's there right now, though, as
> rdtime isn't valid in M-mode on the K210 (our only M-mode platform).
> ---
>  arch/riscv/include/asm/clint.h    | 26 ++++++++++++++++++++++++++
>  arch/riscv/include/asm/timex.h    | 27 +++++++++++++++++++++++++++
>  drivers/clocksource/timer-clint.c | 17 +++++++++++++++++
>  3 files changed, 70 insertions(+)
>  create mode 100644 arch/riscv/include/asm/clint.h
> 
> diff --git a/arch/riscv/include/asm/clint.h b/arch/riscv/include/asm/clint.h
> new file mode 100644
> index 000000000000..0789fd37b40a
> --- /dev/null
> +++ b/arch/riscv/include/asm/clint.h
> @@ -0,0 +1,26 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (C) 2020 Google, Inc
> + */
> +
> +#ifndef _ASM_RISCV_CLINT_H
> +#define _ASM_RISCV_CLINT_H
> +
> +#include <linux/types.h>
> +#include <asm/mmio.h>
> +
> +#ifdef CONFIG_RISCV_M_MODE
> +/*
> + * This lives in the CLINT driver, but is accessed directly by timex.h to avoid
> + * any overhead when accessing the MMIO timer.
> + *
> + * The ISA defines mtime as a 64-bit memory-mapped register that increments at
> + * a constant frequency, but it doesn't define some other constraints we depend
> + * on (most notably ordering constraints, but also some simpler stuff like the
> + * memory layout).  Thus, this is called "clint_time_val" instead of something
> + * like "riscv_mtime", to signify that these non-ISA assumptions must hold.
> + */
> +extern u64 __iomem *clint_time_val;
> +#endif
> +
> +#endif
> diff --git a/arch/riscv/include/asm/timex.h b/arch/riscv/include/asm/timex.h
> index a3fb85d505d4..7f659dda0032 100644
> --- a/arch/riscv/include/asm/timex.h
> +++ b/arch/riscv/include/asm/timex.h
> @@ -10,6 +10,31 @@
>  
>  typedef unsigned long cycles_t;
>  
> +#ifdef CONFIG_RISCV_M_MODE
> +
> +#include <asm/clint.h>
> +
> +#ifdef CONFIG_64BIT
> +static inline cycles_t get_cycles(void)
> +{
> +	return readq_relaxed(clint_time_val);
> +}
> +#else /* !CONFIG_64BIT */
> +static inline u32 get_cycles(void)
> +{
> +	return readl_relaxed(((u32 *)clint_time_val));
> +}
> +#define get_cycles get_cycles
> +
> +static inline u32 get_cycles_hi(void)
> +{
> +	return readl_relaxed(((u32 *)clint_time_val) + 1);
> +}
> +#define get_cycles_hi get_cycles_hi
> +#endif /* CONFIG_64BIT */
> +
> +#else /* CONFIG_RISCV_M_MODE */
> +
>  static inline cycles_t get_cycles(void)
>  {
>  	return csr_read(CSR_TIME);
> @@ -41,6 +66,8 @@ static inline u64 get_cycles64(void)
>  }
>  #endif /* CONFIG_64BIT */
>  
> +#endif /* !CONFIG_RISCV_M_MODE */
> +
>  #define ARCH_HAS_READ_CURRENT_TIMER
>  static inline int read_current_timer(unsigned long *timer_val)
>  {
> diff --git a/drivers/clocksource/timer-clint.c b/drivers/clocksource/timer-clint.c
> index 8eeafa82c03d..d17367dee02c 100644
> --- a/drivers/clocksource/timer-clint.c
> +++ b/drivers/clocksource/timer-clint.c
> @@ -19,6 +19,11 @@
>  #include <linux/interrupt.h>
>  #include <linux/of_irq.h>
>  #include <linux/smp.h>
> +#include <linux/timex.h>
> +
> +#ifndef CONFIG_RISCV_M_MODE
> +#include <asm/clint.h>
> +#endif
>  
>  #define CLINT_IPI_OFF		0
>  #define CLINT_TIMER_CMP_OFF	0x4000
> @@ -31,6 +36,10 @@ static u64 __iomem *clint_timer_val;
>  static unsigned long clint_timer_freq;
>  static unsigned int clint_timer_irq;
>  
> +#ifdef CONFIG_RISCV_M_MODE
> +u64 __iomem *clint_time_val;
> +#endif
> +
>  static void clint_send_ipi(const struct cpumask *target)
>  {
>  	unsigned int cpu;
> @@ -184,6 +193,14 @@ static int __init clint_timer_init_dt(struct device_node *np)
>  	clint_timer_val = base + CLINT_TIMER_VAL_OFF;
>  	clint_timer_freq = riscv_timebase;
>  
> +#ifdef CONFIG_RISCV_M_MODE
> +	/*
> +	 * Yes, that's an odd naming scheme.  time_val is public, but hopefully
> +	 * will die in favor of something cleaner.
> +	 */
> +	clint_time_val = clint_timer_val;
> +#endif
> +
>  	pr_info("%pOFP: timer running at %ld Hz\n", np, clint_timer_freq);
>  
>  	rc = clocksource_register_hz(&clint_clocksource, clint_timer_freq);

With the Kendryte device tree patch that I sent, this works.

Tested-by: Damien Le Moal <damien.lemoal at wdc.com>

-- 
Damien Le Moal
Western Digital Research