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

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Message-ID: <e3d0cb05-268a-6148-7723-fe6b94d96583@wdc.com>
Date:   Thu, 14 Feb 2019 15:49:09 -0800
From:   Atish Patra <atish.patra@....com>
To:     Palmer Dabbelt <palmer@...ive.com>,
        "johan@...nel.org" <johan@...nel.org>
Cc:     "robh@...nel.org" <robh@...nel.org>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "jason@...edaemon.net" <jason@...edaemon.net>,
        "alankao@...estech.com" <alankao@...estech.com>,
        "dmitriy@...-tech.org" <dmitriy@...-tech.org>,
        "schwab@...e.de" <schwab@...e.de>,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "marc.zyngier@....com" <marc.zyngier@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        "anup@...infault.org" <anup@...infault.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [v4 PATCH 8/8] RISC-V: Assign hwcap as per comman capabilities.

On 2/13/19 4:38 PM, Palmer Dabbelt wrote:
> On Wed, 13 Feb 2019 00:44:42 PST (-0800), johan@...nel.org 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.
> 
> Handling malformed device trees by providing a warning and an empty HWCAP seems
> like the right way to go to me.
> 

If I understand you correctly, you prefer following things to be done in 
case of malformed DT.

1. Print a warning message
2. Unset the entire HWCAP
3. Return without processing other harts. This will most likely result 
in panic when user space starts.

Is this correct ?

Regards,
Atish
>>
>> Johan
>