Re: [PATCH v3] RISC-V: Provide the frequency of time CSR via hwprobe

[Punit Agrawal <punit.agrawal@bytedance.com>]

yunhui cui <cuiyunhui@bytedance.com> writes:

> Add punit and sunil in the loop.

Thanks for looping us in.

> On Sat, Jun 22, 2024 at 10:55 AM Yunhui Cui <cuiyunhui@bytedance.com> wrote:
>>
>> From: Palmer Dabbelt <palmer@rivosinc.com>
>>
>> A handful of user-visible behavior is based on the frequency of the
>> time CSR.

It will be helpful to add more context to the commit log - especially
for something that is being exposed in a user visible
interface. Something like below -

    The RISC-V architecture makes a real time counter CSR (via RDTIME
    instruction) available for applications in U-mode but there is no
    architected mechanism for an application to discover the frequency
    the counter is running at. Some applications (e.g., DPDK) use the
    time counter for basic performance analysis as well as fine grained
    time-keeping.

    Add support to the hwprobe system call to export the timer counter
    frequency to code running in U-mode.

With the commit log updated,

Acked-by: Punit Agrawal <punit.agrawal@bytedance.com>

Thanks

>>
>> Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
>> Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
>> Reviewed-by: Evan Green <evan@rivosinc.com>
>> Reviewed-by: Anup Patel <anup@brainfault.org>
>> ---
>>  Documentation/arch/riscv/hwprobe.rst  | 2 ++
>>  arch/riscv/include/asm/hwprobe.h      | 2 +-
>>  arch/riscv/include/uapi/asm/hwprobe.h | 1 +
>>  arch/riscv/kernel/sys_hwprobe.c       | 5 +++++
>>  4 files changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/arch/riscv/hwprobe.rst b/Documentation/arch/riscv/hwprobe.rst
>> index df5045103e73..ec3c99474ed7 100644
>> --- a/Documentation/arch/riscv/hwprobe.rst
>> +++ b/Documentation/arch/riscv/hwprobe.rst
>> @@ -233,3 +233,5 @@ The following keys are defined:
>>
>>  * :c:macro:`RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE`: An unsigned int which
>>    represents the size of the Zicboz block in bytes.
>> +
>> +* :c:macro:`RISCV_HWPROBE_KEY_TIME_CSR_FREQ`: Frequency (in Hz) of `time CSR`.
>> diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
>> index 150a9877b0af..ef01c182af2b 100644
>> --- a/arch/riscv/include/asm/hwprobe.h
>> +++ b/arch/riscv/include/asm/hwprobe.h
>> @@ -8,7 +8,7 @@
>>
>>  #include <uapi/asm/hwprobe.h>
>>
>> -#define RISCV_HWPROBE_MAX_KEY 7
>> +#define RISCV_HWPROBE_MAX_KEY 8
>>
>>  static inline bool riscv_hwprobe_key_is_valid(__s64 key)
>>  {
>> diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
>> index 2fb8a8185e7a..5053a9b18710 100644
>> --- a/arch/riscv/include/uapi/asm/hwprobe.h
>> +++ b/arch/riscv/include/uapi/asm/hwprobe.h
>> @@ -74,6 +74,7 @@ struct riscv_hwprobe {
>>  #define                RISCV_HWPROBE_MISALIGNED_MASK           (7 << 0)
>>  #define RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE    6
>>  #define RISCV_HWPROBE_KEY_MISALIGNED_PERF      7
>> +#define RISCV_HWPROBE_KEY_TIME_CSR_FREQ        8
>>  /* Increase RISCV_HWPROBE_MAX_KEY when adding items. */
>>
>>  /* Flags */
>> diff --git a/arch/riscv/kernel/sys_hwprobe.c b/arch/riscv/kernel/sys_hwprobe.c
>> index e4ec9166339f..3d47edc04a3f 100644
>> --- a/arch/riscv/kernel/sys_hwprobe.c
>> +++ b/arch/riscv/kernel/sys_hwprobe.c
>> @@ -8,6 +8,7 @@
>>  #include <asm/cacheflush.h>
>>  #include <asm/cpufeature.h>
>>  #include <asm/hwprobe.h>
>> +#include <asm/delay.h>
>>  #include <asm/sbi.h>
>>  #include <asm/switch_to.h>
>>  #include <asm/uaccess.h>
>> @@ -227,6 +228,10 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
>>                         pair->value = riscv_cboz_block_size;
>>                 break;
>>
>> +       case RISCV_HWPROBE_KEY_TIME_CSR_FREQ:
>> +               pair->value = riscv_timebase;
>> +               break;
>> +
>>         /*
>>          * For forward compatibility, unknown keys don't fail the whole
>>          * call, but get their element key set to -1 and value set to 0
>> --
>> 2.20.1
>>