Re: [PATCH v3 2/2] Allow for riscv-clock to pick up mmio address.

[Daniel Lezcano <daniel.lezcano@linaro.org>]

On Wed, Apr 23, 2025 at 12:17:37PM +0000, Aleksa Paunovic wrote:
> HTEC Public
> 
> Allow faster rdtime access via GCR.U mtime shadow register on RISC-V
> devices. This feature can be enabled by setting GCRU_TIME_MMIO during configuration.
> 
> Signed-off-by: Aleksa Paunovic <aleksa.paunovic@htecgroup.com>
> ---
>  arch/riscv/include/asm/timex.h    | 59 ++++++++++++++++++++-----------
>  drivers/clocksource/Kconfig       | 12 +++++++
>  drivers/clocksource/timer-riscv.c | 32 +++++++++++++++++
>  3 files changed, 83 insertions(+), 20 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/timex.h b/arch/riscv/include/asm/timex.h
> index a06697846e69..47ad6285b83a 100644
> --- a/arch/riscv/include/asm/timex.h
> +++ b/arch/riscv/include/asm/timex.h
> @@ -7,31 +7,24 @@
>  #define _ASM_RISCV_TIMEX_H
> 
>  #include <asm/csr.h>
> +#include <asm/mmio.h>
> +
> +#include <linux/jump_label.h>
> 
>  typedef unsigned long cycles_t;
> 
> +extern u64 __iomem *riscv_time_val;
> +DECLARE_STATIC_KEY_FALSE(riscv_time_mmio_available);

Please keep it self-encapsulate in the timer-riscv.c

> +#define riscv_time_val riscv_time_val

why ?

>  #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
> +#undef riscv_time_val
> 
> -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 */
> +#define riscv_time_val clint_time_val
> 
>  /*
>   * Much like MIPS, we may not have a viable counter to use at an early point
> @@ -46,22 +39,48 @@ static inline unsigned long random_get_entropy(void)
>  }
>  #define random_get_entropy()   random_get_entropy()
> 
> -#else /* CONFIG_RISCV_M_MODE */
> +#endif
> +
> +static inline long use_riscv_time_mmio(void)
> +{
> +       return IS_ENABLED(CONFIG_RISCV_M_MODE) ||
> +               (IS_ENABLED(CONFIG_GCRU_TIME_MMIO) &&
> +                static_branch_unlikely(&riscv_time_mmio_available));
> +}
> +

Don't use this function to branch when calling get_cycles(). Provide two
versions of the *get_cycles* and initialize with the right one at init time.

> +#ifdef CONFIG_64BIT
> +static inline cycles_t mmio_get_cycles(void)
> +{
> +       return readq_relaxed(riscv_time_val);
> +}
> +#else /* !CONFIG_64BIT */
> +static inline cycles_t mmio_get_cycles(void)
> +{
> +       return readl_relaxed(((u32 *)riscv_time_val));
> +}
> +#endif /* CONFIG_64BIT */
> +
> +static inline u32 mmio_get_cycles_hi(void)
> +{
> +       return readl_relaxed(((u32 *)riscv_time_val) + 1);
> +}
> 
>  static inline cycles_t get_cycles(void)
>  {
> +       if (use_riscv_time_mmio())
> +               return mmio_get_cycles();
>         return csr_read(CSR_TIME);
>  }
>  #define get_cycles get_cycles
> 
>  static inline u32 get_cycles_hi(void)
>  {
> +       if (use_riscv_time_mmio())
> +               return mmio_get_cycles_hi();
>         return csr_read(CSR_TIMEH);
>  }
>  #define get_cycles_hi get_cycles_hi
> 
> -#endif /* !CONFIG_RISCV_M_MODE */
> -
>  #ifdef CONFIG_64BIT
>  static inline u64 get_cycles64(void)
>  {
> diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
> index 487c85259967..0f2bb75564c7 100644
> --- a/drivers/clocksource/Kconfig
> +++ b/drivers/clocksource/Kconfig
> @@ -661,6 +661,18 @@ config CLINT_TIMER
>           This option enables the CLINT timer for RISC-V systems.  The CLINT
>           driver is usually used for NoMMU RISC-V systems.
> 
> +config GCRU_TIME_MMIO
> +       bool "GCR.U timer support for RISC-V platforms"
> +       depends on !RISCV_M_MODE && RISCV
> +       default n
> +       help
> +        Access GCR.U shadow copy of the RISC-V mtime register
> +        on platforms that provide a compatible device, instead of
> +        reading the time CSR. Since reading the time CSR
> +        traps to M mode on certain platforms, this may be more efficient.
> +
> +        If you don't know what to do here, say n.
> +
>  config CSKY_MP_TIMER
>         bool "SMP Timer for the C-SKY platform" if COMPILE_TEST
>         depends on CSKY
> diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
> index 48ce50c5f5e6..4290e4b840f7 100644
> --- a/drivers/clocksource/timer-riscv.c
> +++ b/drivers/clocksource/timer-riscv.c
> @@ -22,6 +22,7 @@
>  #include <linux/io-64-nonatomic-lo-hi.h>
>  #include <linux/interrupt.h>
>  #include <linux/of_irq.h>
> +#include <linux/of_address.h>
>  #include <linux/limits.h>
>  #include <clocksource/timer-riscv.h>
>  #include <asm/smp.h>
> @@ -32,6 +33,13 @@
>  static DEFINE_STATIC_KEY_FALSE(riscv_sstc_available);
>  static bool riscv_timer_cannot_wake_cpu;
> 
> +#if defined(CONFIG_GCRU_TIME_MMIO)
> +DEFINE_STATIC_KEY_FALSE_RO(riscv_time_mmio_available);

static DEFINE_STATIC_KEY_FALSE_RO( ... )

> +EXPORT_SYMBOL(riscv_time_mmio_available);
> +u64 __iomem *riscv_time_val __ro_after_init;
> +EXPORT_SYMBOL(riscv_time_val);
> +#endif
> +
>  static void riscv_clock_event_stop(void)
>  {
>         if (static_branch_likely(&riscv_sstc_available)) {
> @@ -203,6 +211,11 @@ static int __init riscv_timer_init_dt(struct device_node *n)
>         int cpuid, error;
>         unsigned long hartid;
>         struct device_node *child;
> +#if defined(CONFIG_GCRU_TIME_MMIO)
> +       u64 mmio_addr;
> +       u64 mmio_size;
> +       struct device_node *gcru;
> +#endif
> 
>         error = riscv_of_processor_hartid(n, &hartid);
>         if (error < 0) {
> @@ -220,6 +233,25 @@ static int __init riscv_timer_init_dt(struct device_node *n)
>         if (cpuid != smp_processor_id())
>                 return 0;
> 
> +#if defined(CONFIG_GCRU_TIME_MMIO)
> +       gcru = of_find_compatible_node(NULL, NULL, "mti,gcru");
> +       if (gcru) {
> +               if (!of_property_read_reg(gcru, 0, &mmio_addr, &mmio_size)) {
> +                       riscv_time_val = ioremap((long)mmio_addr, mmio_size);
> +                       if (riscv_time_val) {
> +                               pr_info("Using mmio time register at 0x%llx\n",
> +                                       mmio_addr);
> +                               static_branch_enable(
> +                                       &riscv_time_mmio_available);
> +                       } else {
> +                               pr_warn("Unable to use mmio time at 0x%llx\n",
> +                                       mmio_addr);
> +                       }
> +                       of_node_put(gcru);
> +               }
> +       }
> +#endif
> +
>         child = of_find_compatible_node(NULL, NULL, "riscv,timer");
>         if (child) {
>                 riscv_timer_cannot_wake_cpu = of_property_read_bool(child,
> --
> 2.34.1

-- 

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