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

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Date:   Tue, 21 Jul 2020 19:03:04 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Jordan Niethe <jniethe5@...il.com>, 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



On 7/21/20 5:06 PM, Michael Ellerman wrote:
> 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.

Agreed. By adding a comment, we can remove those macros. Will change it.

Thanks,
Ravi