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

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Date:   Wed, 6 Jul 2022 12:00:33 +0800
From:   Qi Hu <huqi@...ngson.cn>
To:     Xi Ruoyao <xry111@...111.site>, Huacai Chen <chenhuacai@...nel.org>
Cc:     WANG Xuerui <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        loongarch@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] LoongArch: Clean useless vcsr in loongarch_fpu.


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*).

- Flush to zero(MSACSR.FS) is removed and not supported.

- If you use "movfcsr2gr" to read the fcsr16, the value is *UNDEFINED*.