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

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Message-ID: <CAK9=C2Xoi7zW0evH-q15699vRqQ1UY72C_Z3UpYHMbkgL+efUg@mail.gmail.com>
Date:   Wed, 27 Jul 2022 17:23:24 +0530
From:   Anup Patel <apatel@...tanamicro.com>
To:     Ben Dooks <ben.dooks@...ethink.co.uk>
Cc:     Anup Patel <anup@...infault.org>,
        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 Wed, Jul 27, 2022 at 3:42 PM Ben Dooks <ben.dooks@...ethink.co.uk> wrote:
>
> 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?

Well, we are not simplifying anything by moving from a "#if" ladder
to "if ()" ladder. Also, I don't see how the "#if" ladder will grow over
time.

Regards,
Anup

>
> >>
> >>> +#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
>
> https://www.codethink.co.uk/privacy.html