[U-Boot] [PATCH] riscv: add Kconfig entries for the F and D ISA extensions support

[Greentime Hu]

Hi Bin,
On Tue, May 28, 2019 at 05:58:47PM +0800, Bin Meng wrote:
> Hi Greentime,
> 
> On Tue, May 28, 2019 at 3:54 PM Greentime Hu <greentime at andestech.com> wrote:
> >
> > Hi Bin,
> > On Fri, May 24, 2019 at 05:40:08PM +0800, Bin Meng wrote:
> > > Hi Eric,
> > >
> > > On Fri, May 24, 2019 at 4:04 PM Eric Lin <dslin1010 at gmail.com> wrote:
> > > >
> > > > Hi Bin
> > > >
> > > > Bin Meng <bmeng.cn at gmail.com> ??? 2019???5???22??? ?????? ??????5:25?????????
> > > > >
> > > > > Hi Eric,
> > > > >
> > > > > On Wed, May 22, 2019 at 4:23 PM <tesheng at andestech.com> wrote:
> > > > > >
> > > > > > Hi Bin,
> > > > > >
> > > > > > > -----Original Message-----
> > > > > > > From: Bin Meng [mailto:bmeng.cn at gmail.com]
> > > > > > > Sent: Tuesday, May 21, 2019 3:56 PM
> > > > > > > To: Eric Te-Sheng Lin(?????????)
> > > > > > > Cc: U-Boot Mailing List; Lukas Auer; Anup Patel; Rick Jian-Zhi Chen(?????????);
> > > > > > > Greentime Ying-Han Hu(?????????); dslin1010 at gmail.com
> > > > > > > Subject: Re: [PATCH] riscv: add Kconfig entries for the F and D ISA extensions
> > > > > > > support
> > > > > > >
> > > > > > > Hi Eric,
> > > > > > >
> > > > > > > On Tue, May 21, 2019 at 3:18 PM Eric Lin <tesheng at andestech.com> wrote:
> > > > > > > >
> > > > > > > > This patch add Kconfig entries for the F (Single-Precision)
> > > > > > >
> > > > > > > adds
> > > > > > >
> > > > > >
> > > > > > OK I'll correct it as adds
> > > > > >
> > > > > > > > and D (Double-Precision) floating point instruction-set extensions.
> > > > > > > >
> > > > > > >
> > > > > > > Could you please provide reason that why U-Boot has to be compiled using F/D
> > > > > > > extension?
> > > > > > >
> > > > > >
> > > > > > Cause on AE350 platform, we have two different kinds of toolchain v5d (support I/M/A/C/F/D ISA) and
> > > > > > v5 (support I/M/A/C ISA). If we use the v5d toolchain to build U-Boot it will build fail, so we would like to add F/D extension on U-Boot.
> > > > >
> > > > > I don't understand. What difference do these two toochains have? Isn't
> > > > > the v5d toolchain's default -march string be pre-configured to imafd?
> > > > > But even if the toolchain is pre-configured to generate fd
> > > > > instruction, I think it can be override by the compiler flags. Can you
> > > > > please share the details of the toolchain you used? I suspect you have
> > > > > to fix your toolchain, not U-Boot.
> > > > >
> > > >
> > > > It's seems the ABI issue. Because our toolchain don't support
> > > > multilib, the v5d toolchain libraries ABI is ilp64d.
> > > > If I use the v5d toolchain to build U-Boot with -mabi=lp64, it will
> > > > get link error as below:
> > >
> > > Then please fix your compiler. Passing -mabi=lp64 to compiler is
> > > definitely correct.
> >
> >  One of our customer ever used floating point in their project. They want
> >  to calculate something with floating point instructions in u-Boot before
> >  entering kernel.
> 
> This is a new use case. For other architectures like x86, we don't
> allow U-Boot to be compiled using hardware float ABI. Why can't they
> use software floating point instead?

It's because they'd like to calculate something very quickly. It may just be
done in few cycles with floating point instructions and it will be very useful
for their usages instead of using soft-float.

That's the reason they ask us to provide such kind of CONFIG and we think it
might also be helpful for users who are trying to do some mathematics with
floating point instructions in u-Boot.

Thanks. :)

> >
> >  IMHO, we may consider this case for user to use -mabi=ilp64d.
> >  This CONFIG will be useful for them to compile their codes and linking together
> >  with u-Boot.
> >
> >  Thanks :)
> 
> Regards,
> Bin