linux-kernel - Re: [PATCH v2] LoongArch: Clean useless vcsr in loongarch

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Date:   Thu, 7 Jul 2022 13:42:59 +0800
From:   Qi Hu <huqi@...ngson.cn>
To:     WANG Xuerui <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Xi Ruoyao <xry111@...111.site>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     loongarch@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] LoongArch: Clean useless vcsr in loongarch_fpu.


On 2022/7/7 11:05, WANG Xuerui wrote:
> On 2022/7/7 09:29, Qi Hu wrote:
>>
>> On 2022/7/7 04:49, Jiaxun Yang wrote:
>>>
>>> 在2022年7月6日七月 上午5:00，Qi Hu写道：
>>>> On 2022/7/6 10:51, Xi Ruoyao wrote:
>>>>> On Wed, 2022-07-06 at 10:35 +0800, Huacai Chen wrote:
>>>>>
>>>>>> Maybe Xuerui and Ruoyao have some misunderstanding. LSX/LASX will
>>>>>> surely be upstream, this has nothing to do with cleanup VCSR16.
>>>>>> Because FP/LSX/LASX share the same control bits in FCSR now.
>>>>> My guess:
>>>>>
>>>>> Almost all behavior of vector unit is controlled by FCSR (for 
>>>>> example,
>>>>> the rounding of both FPU and vector unit should be controlled by FCSR
>>>>> altogether), except one bit similar to the bit 24 of MSACSR 
>>>>> ("flush to
>>>>> zero") is in VCSR [^1].  And "flush to zero" is not really useful 
>>>>> so it
>>>>> will be removed in 3A6000, and we'll not use it for 3A5000.
>>>> Actually, flush to zero has been removed in 3A5000.
>>>>> [^1]: A more bold guess: the hardware engineers could have just said
>>>>> "let's wire this register called MSACSR in GS464V as FCSR16/VCSR in
>>>>> LA464, maybe it will be useful and who knows?"  But now in 
>>>>> practice it's
>>>>> not useful.
>>>>>
>>>>> Am I correct?
>>>> The hardware(LA464) has removed the vcsr("has but not use" is
>>>> incorrect), and here are some details:
>>>>
>>>> - For all FP operations, including LSX/LASX, they are controlled by
>>>> fcsr0/1/2/3.
>>>>
>>>> - For LSX/LASX other operations, they are *not* controlled by any 
>>>> other
>>>> CSR now. And fcsr16 to fcsr31 are reserved to control these operations
>>>> (now they are *undefined*).
>>> Sorry but what do you meant by “these” here?
>> "These operations" means "LSX/LASX other operations", except its 
>> floating-point operations.
>
> This is getting hard to follow. Assuming the expression "LSX/LASX 
> other operations" is Chinglish (it's certainly not proper English), I 
> think you mean "the non-FP operations belonging to LSX/LASX" here right?
That's right.
>
> And it's strange, that these ops do exist in LSX/LASX, hence also 
> present in 3A5000, but the control bits are undefined. How come this 
> is even possible?
The code is redundant, actually. Reading or writing the fcsr16 do not 
have any effect on LSX/LASX.