linux-kernel - Re: [PATCH v3 5/6] MIPS: Loongson64: SMP: Fix up play

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Message-ID: <cf187478-8f9d-b321-3ea2-4fbaf70ed330@loongson.cn>
Date:   Wed, 4 Nov 2020 15:18:08 +0800
From:   Jinyang He <hejinyang@...ngson.cn>
To:     Jiaxun Yang <jiaxun.yang@...goat.com>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Huacai Chen <chenhc@...ote.com>
Cc:     linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Lu Zeng <zenglu@...ngson.cn>, Jun Yi <yijun@...ngson.cn>
Subject: Re: [PATCH v3 5/6] MIPS: Loongson64: SMP: Fix up play_dead jump
 indicator

On 11/04/2020 03:04 PM, Jiaxun Yang wrote:

>
>
> 在 2020/11/4 14:31, Jinyang He 写道:
>> Hi, all,
>>
>> On 11/03/2020 03:12 PM, Tiezhu Yang wrote:
>>> In play_dead function, the whole 64-bit PC mailbox was used as a 
>>> indicator
>>> to determine if the master core had written boot jump information.
>>>
>>> However, after we introduced CSR mailsend, the hardware will not 
>>> guarante
>>> an atomic write for the 64-bit PC mailbox. Thus we have to use the 
>>> lower
>>> 32-bit which is written at the last as the jump indicator instead.
>>>
>>> Signed-off-by: Lu Zeng <zenglu@...ngson.cn>
>>> Signed-off-by: Jun Yi <yijun@...ngson.cn>
>>> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
>>> ---
>>>
>>> v2: No changes
>>> v3: Update the commit message and comment
>>>
>>>   arch/mips/loongson64/smp.c | 3 ++-
>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/mips/loongson64/smp.c b/arch/mips/loongson64/smp.c
>>> index 736e98d..aa0cd72 100644
>>> --- a/arch/mips/loongson64/smp.c
>>> +++ b/arch/mips/loongson64/smp.c
>>> @@ -764,9 +764,10 @@ static void loongson3_type3_play_dead(int 
>>> *state_addr)
>>>           "1: li    %[count], 0x100             \n" /* wait for init 
>>> loop */
>>>           "2: bnez  %[count], 2b                \n" /* limit mailbox 
>>> access */
>>>           "   addiu %[count], -1                \n"
>>> -        "   ld    %[initfunc], 0x20(%[base])  \n" /* get PC via 
>>> mailbox */
>> I have some confusion here. Play_dead CPUs is always brought up by 
>> cpu_up().
>> On Loongson64, it calls loongson3_boot_secondary(). The value of 
>> startargs[0]
>> is the address of smp_bootstrap() which is in CKSEG0 and a constant 
>> after the
>> kernel is compiled. That means its value likes 0xffffffff8... and 
>> only the low
>> 32bit is useful. As "lw" is sign-extended, could we replace "ld" with 
>> "lw" simply?
>
> Hi Jinyang,
>
> I'd prefer not to do so. In future we may have kernel running in other 
> spaces,
> (e.g. xkphys), and there is no reason to add a barrier on that without 
> actual benefit.
> I had check PMON firmware and it's also loading the full 64-bit address.
>
> Also to keep consistent, mailbox writing part needs to be refined to 
> match the
> behavior of reading. Otherwise other readers will be confused.
>
> Thus leaving it as is looks much more reasonable.

OK, the current code looks better,
please ignore my comment, sorry for the noise.

Thanks,
Jinyang

>
> Thanks.
>
> - Jiaxun
>
>>
>> Thanks,
>> Jinyang
>>> +        "   lw    %[initfunc], 0x20(%[base])  \n" /* check lower 
>>> 32-bit as jump indicator */
>>>           "   beqz  %[initfunc], 1b             \n"
>>>           "   nop                               \n"
>>> +        "   ld    %[initfunc], 0x20(%[base])  \n" /* get PC (whole 
>>> 64-bit) via mailbox */
>>>           "   ld    $sp, 0x28(%[base])          \n" /* get SP via 
>>> mailbox */
>>>           "   ld    $gp, 0x30(%[base])          \n" /* get GP via 
>>> mailbox */
>>>           "   ld    $a1, 0x38(%[base])          \n"