linux-kernel - Re: [PATCH V2] arm64/cpufeature: Add get_arm64_ftr_reg

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Message-ID: <ca38b2c0-533f-9b98-46a2-37ba8bf21d83@arm.com>
Date:   Wed, 27 May 2020 07:56:30 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mark Brown <broonie@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] arm64/cpufeature: Add get_arm64_ftr_reg_nowarn()



On 05/27/2020 01:16 AM, Will Deacon wrote:
> On Tue, May 26, 2020 at 04:01:35PM +0100, Catalin Marinas wrote:
>> On Tue, May 26, 2020 at 07:09:13PM +0530, Anshuman Khandual wrote:
>>> @@ -632,8 +654,6 @@ static void __init init_cpu_ftr_reg(u32 sys_reg, u64 new)
>>>  	const struct arm64_ftr_bits *ftrp;
>>>  	struct arm64_ftr_reg *reg = get_arm64_ftr_reg(sys_reg);
>>>  
>>> -	BUG_ON(!reg);
>>> -
>>>  	for (ftrp = reg->ftr_bits; ftrp->width; ftrp++) {
>>>  		u64 ftr_mask = arm64_ftr_mask(ftrp);
>>>  		s64 ftr_new = arm64_ftr_value(ftrp, new);
>>> @@ -762,7 +782,6 @@ static int check_update_ftr_reg(u32 sys_id, int cpu, u64 val, u64 boot)
>>>  {
>>>  	struct arm64_ftr_reg *regp = get_arm64_ftr_reg(sys_id);
>>>  
>>> -	BUG_ON(!regp);
>>>  	update_cpu_ftr_reg(regp, val);
>>>  	if ((boot & regp->strict_mask) == (val & regp->strict_mask))
>>>  		return 0;
>>> @@ -776,9 +795,6 @@ static void relax_cpu_ftr_reg(u32 sys_id, int field)
>>>  	const struct arm64_ftr_bits *ftrp;
>>>  	struct arm64_ftr_reg *regp = get_arm64_ftr_reg(sys_id);
>>>  
>>> -	if (WARN_ON(!regp))
>>> -		return;
>>
>> I think Will wanted an early return in all these functions not just
>> removing the BUG_ON(). I'll let him clarify.
> 
> Yes, the callers need to check the pointer and return early.

Sure, will do. But for check_update_ftr_reg(), a feature register search
failure should be treated as a success (0) or a failure (1). What should
it return ? Seems bit tricky, as there are good reasons to go either way.