linux-kernel - Re: [PATCH v2] RISC-V: Add mvendorid, marchid, and mimpid to /proc/cpuinfo output

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Date:   Wed, 27 Jul 2022 11:12:14 +0100
From:   Ben Dooks <ben.dooks@...ethink.co.uk>
To:     Anup Patel <anup@...infault.org>
Cc:     Anup Patel <apatel@...tanamicro.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        Heinrich Schuchardt <heinrich.schuchardt@...onical.com>,
        Atish Patra <atishp@...shpatra.org>,
        linux-riscv@...ts.infradead.org, Nikita Shubin <n.shubin@...ro.com>
Subject: Re: [PATCH v2] RISC-V: Add mvendorid, marchid, and mimpid to
 /proc/cpuinfo output

On 27/07/2022 11:06, Anup Patel wrote:
> On Wed, Jul 27, 2022 at 2:25 PM Ben Dooks <ben.dooks@...ethink.co.uk> wrote:
>>
>> On 27/07/2022 05:38, Anup Patel wrote:
>>> Identifying the underlying RISC-V implementation can be important
>>> for some of the user space applications. For example, the perf tool
>>> uses arch specific CPU implementation id (i.e. CPUID) to select a
>>> JSON file describing custom perf events on a CPU.
>>>
>>> Currently, there is no way to identify RISC-V implementation so we
>>> add mvendorid, marchid, and mimpid to /proc/cpuinfo output.
>>>
>>> Signed-off-by: Anup Patel <apatel@...tanamicro.com>
>>> Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
>>> Tested-by: Nikita Shubin <n.shubin@...ro.com>
>>> ---
>>> Changes since v1:
>>>    - Use IS_ENABLED() to check CONFIG defines
>>>    - Added RB and TB tags in commit description
>>> ---
>>>    arch/riscv/kernel/cpu.c | 51 +++++++++++++++++++++++++++++++++++++++++
>>>    1 file changed, 51 insertions(+)
>>>
>>> diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
>>> index fba9e9f46a8c..04bcc91c91ea 100644
>>> --- a/arch/riscv/kernel/cpu.c
>>> +++ b/arch/riscv/kernel/cpu.c
>>> @@ -3,10 +3,13 @@
>>>     * Copyright (C) 2012 Regents of the University of California
>>>     */
>>>
>>> +#include <linux/cpu.h>
>>>    #include <linux/init.h>
>>>    #include <linux/seq_file.h>
>>>    #include <linux/of.h>
>>> +#include <asm/csr.h>
>>>    #include <asm/hwcap.h>
>>> +#include <asm/sbi.h>
>>>    #include <asm/smp.h>
>>>    #include <asm/pgtable.h>
>>>
>>> @@ -64,6 +67,50 @@ int riscv_of_parent_hartid(struct device_node *node)
>>>    }
>>>
>>>    #ifdef CONFIG_PROC_FS
>>> +
>>> +struct riscv_cpuinfo {
>>> +     unsigned long mvendorid;
>>> +     unsigned long marchid;
>>> +     unsigned long mimpid;
>>> +};
>>> +static DEFINE_PER_CPU(struct riscv_cpuinfo, riscv_cpuinfo);
>>> +
>>> +static int riscv_cpuinfo_starting(unsigned int cpu)
>>> +{
>>> +     struct riscv_cpuinfo *ci = this_cpu_ptr(&riscv_cpuinfo);
>>> +
>>> +#if IS_ENABLED(CONFIG_RISCV_SBI)
>>> +     ci->mvendorid = sbi_spec_is_0_1() ? 0 : sbi_get_mvendorid();
>>> +     ci->marchid = sbi_spec_is_0_1() ? 0 : sbi_get_marchid();
>>> +     ci->mimpid = sbi_spec_is_0_1() ? 0 : sbi_get_mimpid();
>>
>> how about:
>>
>>          if (IS_ENABLED(CONFIG_RISCV_SBI)) {
>>                  ...
>>          } ... {
>>
>> or maybe even:
>>
>>
>>          if (IS_ENABLED(CONFIG_RISCV_SBI)) {
>>                  if (sbi_spec_is_0_1()) {
>>                          ...
>>                  }
>>          } ... {
>>
>> would mean better compile coverage (at the slight exepnese of
>> having "false" sbi_spec_is_0_1() implemenation
> 
> Most of the sbi_xyz() functions are not available for NoMMU
> kernel so using "if (IS_ENABLED())" results in compile error.

How about defining "false" versions for no-mmu case and try
and avoid these #if mountains?

>>
>>> +#elif IS_ENABLED(CONFIG_RISCV_M_MODE)
>>> +     ci->mvendorid = csr_read(CSR_MVENDORID);
>>> +     ci->marchid = csr_read(CSR_MARCHID);
>>> +     ci->mimpid = csr_read(CSR_MIMPID);
>>> +#else
>>> +     ci->mvendorid = 0;
>>> +     ci->marchid = 0;
>>> +     ci->mimpid = 0;
>>> +#endif
>>
>> Would it be easier to zero out all the fields first and then fill them
>> in if supported?
> 
> Clearing out fields before "#if" ladder results in dead assignments.

Not sure which is worse here, the #if ladder or some possibly dead
assignments.

-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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