lkml.org 
[lkml]   [2022]   [Jul]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH v2] LoongArch: Clean useless vcsr in loongarch_fpu.
From

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.

\
 
 \ /
  Last update: 2022-07-07 07:44    [W:0.095 / U:1.036 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site