Re: [PATCH v2] LoongArch: Clean useless vcsr in loongarch_fpu.

[Qi Hu]

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.