linux-kernel - Re: [v4 PATCH 8/8] RISC-V: Assign hwcap as per comman capabilities.

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Date:   Wed, 13 Feb 2019 11:59:55 -0800
From:   Atish Patra <atish.patra@....com>
To:     Johan Hovold <johan@...nel.org>
Cc:     Rob Herring <robh@...nel.org>, Albert Ou <aou@...s.berkeley.edu>,
        Jason Cooper <jason@...edaemon.net>,
        Alan Kao <alankao@...estech.com>,
        Dmitriy Cherkasov <dmitriy@...-tech.org>,
        Andreas Schwab <schwab@...e.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Anup Patel <anup@...infault.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [v4 PATCH 8/8] RISC-V: Assign hwcap as per comman capabilities.

On 2/13/19 12:44 AM, Johan Hovold wrote:
> On Tue, Feb 12, 2019 at 11:58:10AM -0800, Atish Patra wrote:
>> On 2/12/19 3:25 AM, Johan Hovold wrote:
>>> On Tue, Feb 12, 2019 at 03:10:12AM -0800, Atish Patra wrote:
>>>> Currently, we set hwcap based on first valid hart from DT. This may not
>>>> be correct always as that hart might not be current booting cpu or may
>>>> have a different capability.
>>>>
>>>> Set hwcap as the capabilities supported by all possible harts with "okay"
>>>> status.
>>>>
>>>> Signed-off-by: Atish Patra <atish.patra@....com>
>>>> ---
>>>>    arch/riscv/kernel/cpufeature.c | 41 ++++++++++++++++++++++-------------------
>>>>    1 file changed, 22 insertions(+), 19 deletions(-)
>>>>
>>>> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
>>>> index e7a4701f..a1e4fb34 100644
>>>> --- a/arch/riscv/kernel/cpufeature.c
>>>> +++ b/arch/riscv/kernel/cpufeature.c
>>>> @@ -20,6 +20,7 @@
>>>>    #include <linux/of.h>
>>>>    #include <asm/processor.h>
>>>>    #include <asm/hwcap.h>
>>>> +#include <asm/smp.h>
>>>>    
>>>>    unsigned long elf_hwcap __read_mostly;
>>>>    #ifdef CONFIG_FPU
>>>> @@ -42,28 +43,30 @@ void riscv_fill_hwcap(void)
>>>>    
>>>>    	elf_hwcap = 0;
>>>>    
>>>> -	/*
>>>> -	 * We don't support running Linux on hertergenous ISA systems.  For
>>>> -	 * now, we just check the ISA of the first "okay" processor.
>>>> -	 */
>>>>    	for_each_of_cpu_node(node) {
>>>> -		if (riscv_of_processor_hartid(node) >= 0)
>>>> -			break;
>>>> -	}
>>>> -	if (!node) {
>>>> -		pr_warn("Unable to find \"cpu\" devicetree entry\n");
>>>> -		return;
>>>> -	}
>>>> +		unsigned long this_hwcap = 0;
>>>>    
>>>> -	if (of_property_read_string(node, "riscv,isa", &isa)) {
>>>> -		pr_warn("Unable to find \"riscv,isa\" devicetree entry\n");
>>>> -		of_node_put(node);
>>>> -		return;
>>>> -	}
>>>> -	of_node_put(node);
>>>> +		if (riscv_of_processor_hartid(node) < 0)
>>>> +			continue;
>>>>
>>
>>>> -	for (i = 0; i < strlen(isa); ++i)
>>>> -		elf_hwcap |= isa2hwcap[(unsigned char)(isa[i])];
>>>> +		if (of_property_read_string(node, "riscv,isa", &isa)) {
>>>> +			pr_warn("Unable to find \"riscv,isa\" devicetree entry\n");
>>>> +			return;
>>>
>>> Did you want "continue" here to continue processing the other harts?
>>
>> Hmm. If a cpu node doesn't have isa in DT, that means DT is wrong. A
>> "continue" here will let user space use other harts just with a warning
>> message?
>>
>> Returning here will not set elf_hwcap which forces the user to fix the
>> DT. I am not sure what should be the defined behavior in this case.
>>
>> Any thoughts ?
> 
> The problem is that the proposed code might still set elf_hwcap -- it
> all depends on the order of the hart nodes in dt (i.e. it will only be
> left unset if the first node is malformed).
> 
> For that reason, I'd say it's better to either bail out (hard or at
> least with elf_hwcap unset) or to continue processing the other nodes.
> 
> The former might break current systems with malformed dt, though.
> 
> And since the harts are expected to have the same ISA, continuing the
> processing while warning and ignoring the malformed node might be
> acceptable.
> 

ok. I will change it to continue unless somebody else has objection.

Thanks for the review.

Regards,
Atish
> Johan
>