linux-kernel - Re: [PATCH v4 09/10] powerpc/watchpoint: Return available watchpoints dynamically

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Date:   Tue, 21 Jul 2020 21:36:54 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
        Jordan Niethe <jniethe5@...il.com>
Cc:     mikey@...ling.org, apopple@...ux.ibm.com,
        Paul Mackerras <paulus@...ba.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@....fr>,
        naveen.n.rao@...ux.vnet.ibm.com, peterz@...radead.org,
        jolsa@...nel.org, oleg@...hat.com, fweisbec@...il.com,
        mingo@...nel.org, pedromfc@...ibm.com, miltonm@...ibm.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Subject: Re: [PATCH v4 09/10] powerpc/watchpoint: Return available watchpoints dynamically

Ravi Bangoria <ravi.bangoria@...ux.ibm.com> writes:
> On 7/20/20 9:12 AM, Jordan Niethe wrote:
>> On Fri, Jul 17, 2020 at 2:11 PM Ravi Bangoria
>> <ravi.bangoria@...ux.ibm.com> wrote:
>>>
>>> So far Book3S Powerpc supported only one watchpoint. Power10 is
>>> introducing 2nd DAWR. Enable 2nd DAWR support for Power10.
>>> Availability of 2nd DAWR will depend on CPU_FTR_DAWR1.
>>>
>>> Signed-off-by: Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
>>> ---
>>>   arch/powerpc/include/asm/cputable.h      | 4 +++-
>>>   arch/powerpc/include/asm/hw_breakpoint.h | 5 +++--
>>>   2 files changed, 6 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/powerpc/include/asm/cputable.h b/arch/powerpc/include/asm/cputable.h
>>> index 3445c86e1f6f..36a0851a7a9b 100644
>>> --- a/arch/powerpc/include/asm/cputable.h
>>> +++ b/arch/powerpc/include/asm/cputable.h
>>> @@ -633,7 +633,9 @@ enum {
>>>    * Maximum number of hw breakpoint supported on powerpc. Number of
>>>    * breakpoints supported by actual hw might be less than this.
>>>    */
>>> -#define HBP_NUM_MAX    1
>>> +#define HBP_NUM_MAX    2
>>> +#define HBP_NUM_ONE    1
>>> +#define HBP_NUM_TWO    2

>> I wonder if these defines are necessary - has it any advantage over
>> just using the literal?
>
> No, not really. Initially I had something like:
>
> #define HBP_NUM_MAX    2
> #define HBP_NUM_P8_P9  1
> #define HBP_NUM_P10    2
>
> But then I thought it's also not right. So I made it _ONE and _TWO.
> Now the function that decides nr watchpoints dynamically (nr_wp_slots)
> is in different file, I thought to keep it like this so it would be
> easier to figure out why _MAX is 2.

I don't think it makes anything clearer.

I had to stare at it thinking there was some sort of mapping or
indirection going on, before I realised it's just literally the number
of breakpoints.

So please just do:

static inline int nr_wp_slots(void)
{
       return cpu_has_feature(CPU_FTR_DAWR1) ? 2 : 1;
}

If you think HBP_NUM_MAX needs explanation then do that with a comment,
it can refer to nr_wp_slots() if that's helpful.

cheers