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

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Date:   Thu, 7 Jul 2022 09:29:37 +0800
From:   Qi Hu <huqi@...ngson.cn>
To:     Jiaxun Yang <jiaxun.yang@...goat.com>,
        Xi Ruoyao <xry111@...111.site>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     Xuerui Wang <kernel@...0n.name>, 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 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.
> If it means LSX/LASX, are you trying to say that future chip’s LSX/LASX won’t be
> compatible with present 3A5000? As your said fcsr16 and fcsr31 are undefined
> for now.
>
> Thanks
> -

"not compatible" is incorrect. If future chips add new features to 
define and use these registers, some bits in CPUCFG should be set, like 
CPUID in X86.

And at that time, if the applications do not use these new features(like 
some old apps), they can run at *default* state which is the same as 
current 3A5000.

So, "reserved" is just to prepare for adding something, not "incompatible".

Thanks.

>> - Flush to zero(MSACSR.FS) is removed and not supported.
>>
>> - If you use "movfcsr2gr" to read the fcsr16, the value is *UNDEFINED*.