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

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Message-ID: <a439dff1-2a61-8378-d510-af754ed746ec@linux.ibm.com>
Date:   Tue, 21 Jul 2020 13:45:58 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
To:     Jordan Niethe <jniethe5@...il.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>, mikey@...ling.org,
        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 <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


>>>> @@ -46,7 +47,7 @@ struct arch_hw_breakpoint {
>>>>
>>>>    static inline int nr_wp_slots(void)
>>>>    {
>>>> -       return HBP_NUM_MAX;
>>>> +       return cpu_has_feature(CPU_FTR_DAWR1) ? HBP_NUM_TWO : HBP_NUM_ONE;
>>> So it'd be something like:
>>> +       return cpu_has_feature(CPU_FTR_DAWR1) ? HBP_NUM_MAX : 1;
>>> But thinking that there might be more slots added in the future, it
>>> may be better to make the number of slots a variable that is set
>>> during the init and then have this function return that.
>>
>> Not sure I follow. What do you mean by setting number of slots a
>> variable that is set during the init?
> Sorry I was unclear there.
> I was just looking and saw arm also has a variable number of hw breakpoints.
> If we did something like how they handle it, it might look something like:
> 
> static int num_wp_slots __ro_after_init;
> 
> int nr_wp_slots(void) {
>      return num_wp_slots;
> }
> 
> static int __init arch_hw_breakpoint_init(void) {
>      num_wp_slots = work out how many wp_slots
> }
> arch_initcall(arch_hw_breakpoint_init);
> 
> Then we wouldn't have to calculate everytime nr_wp_slots() is called.
> In the future if more wp's are added nr_wp_slots() will get more complicated.
> But just an idea, feel free to ignore.

Ok I got the idea. But ARM arch_hw_breakpoint_init() is much more complex
compared to our nr_wp_slots(). I don't see any benefit by making our code
like ARM.

Thanks for the idea though :)
Ravi