[U-Boot] [PATCH v5 4/4] riscv: Remove redundant a2 store on DRAM base in start.S

[Bin Meng <bmeng.cn@gmail.com>]

Hi Rick,

On Tue, Nov 27, 2018 at 4:43 PM Rick Chen <rickchen36@gmail.com> wrote:
>
> Anup Patel <anup@brainfault.org> 於 2018年11月27日 週二 下午3:56寫道：
> >
> > On Tue, Nov 27, 2018 at 12:30 PM Rick Chen <rickchen36@gmail.com> wrote:
> > >
> > > Anup Patel <anup@brainfault.org> 於 2018年11月27日 週二 下午2:40寫道：
> > > >
> > > > On Tue, Nov 27, 2018 at 12:00 PM Rick Chen <rickchen36@gmail.com> wrote:
> > > > >
> > > > > Anup Patel <anup@brainfault.org> 於 2018年11月27日 週二 下午2:14寫道：
> > > > > >
> > > > > >
> > > > > >
> > > > > > On Tue, 27 Nov, 2018, 11:38 AM Rick Chen <rickchen36@gmail.com wrote:
> > > > > >>
> > > > > >> Anup Patel <anup@brainfault.org> 於 2018年11月27日 週二 下午1:47寫道：
> > > > > >> >
> > > > > >> > On Tue, Nov 27, 2018 at 11:14 AM Anup Patel <anup@brainfault.org> wrote:
> > > > > >> > >
> > > > > >> > > On Tue, Nov 27, 2018 at 10:50 AM Rick Chen <rickchen36@gmail.com> wrote:
> > > > > >> > > >
> > > > > >> > > > Anup Patel <anup@brainfault.org> 於 2018年11月27日 週二 上午11:28寫道：
> > > > > >> > > > >
> > > > > >> > > > > On Tue, Nov 27, 2018 at 8:50 AM Rick Chen <rickchen36@gmail.com> wrote:
> > > > > >> > > > > >
> > > > > >> > > > > > > > > > Currently, the RISC-V U-Boot is saving a2 register at
> > > > > >> > > > > > > > > > CONFIG_SYS_DRAM_BASE in start.S which does not make sense because
> > > > > >> > > > > > > > > > there is no information passed by previous booting stage in a2
> > > > > >> > > > > > > > > > register.
> > > > > >> > > > > > > > > >
> > > > > >> > > > > > > > > > This patch removes redundant a2 store on DRAM base.
> > > > > >> > > > > > > > > >
> > > > > >> > > > > > > > > > Signed-off-by: Anup Patel <anup@brainfault.org>
> > > > > >> > > > > > > > > > ---
> > > > > >> > > > > > > > > >  arch/riscv/cpu/start.S | 2 --
> > > > > >> > > > > > > > > >  1 file changed, 2 deletions(-)
> > > > > >> > > > > > > > > >
> > > > > >> > > > > > > > > > diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S index
> > > > > >> > > > > > > > > > 704190f946..e4276e8e19 100644
> > > > > >> > > > > > > > > > --- a/arch/riscv/cpu/start.S
> > > > > >> > > > > > > > > > +++ b/arch/riscv/cpu/start.S
> > > > > >> > > > > > > > > > @@ -38,8 +38,6 @@ _start:
> > > > > >> > > > > > > > > >         mv      s0, a0
> > > > > >> > > > > > > > > >         mv      s1, a1
> > > > > >> > > > > > > > > >
> > > > > >> > > > > > > > > > -       li      t0, CONFIG_SYS_SDRAM_BASE
> > > > > >> > > > > > > > > > -       SREG    a2, 0(t0)
> > > > > >> > > > > > > > > >         la      t0, trap_entry
> > > > > >> > > > > > > > > >  #ifdef CONFIG_RISCV_SMODE
> > > > > >> > > > > > > > > >         csrw    stvec, t0
> > > > > >> > > > > > > > > > --
> > > > > >> > > > > > > > >
> > > > > >> > > > > > > > > This is weird. I remember these two lines were already removed by
> > > > > >> > > > > > > > > Lukas's patch series before? Did not have time to dig out the history
> > > > > >> > > > > > > > > though.
> > > > > >> > > > > > > > >
> > > > > >> > > > > >
> > > > > >> > > > > > > > > Regards,
> > > > > >> > > > > > > > > Bin
> > > > > >> > > > > > > >
> > > > > >> > > > > > > > You are correct, however I removed it again, because I did not want to break
> > > > > >> > > > > > > > Rick's board. He did add a commit to the last pull request that removes these
> > > > > >> > > > > > > > two lines and adjusts his board accordingly, but it is not in the current one.
> > > > > >> > > > > > > >
> > > > > >> > > > > >
> > > > > >> > > > > > Hi Likas
> > > > > >> > > > > >
> > > > > >> > > > > > Thanks for your explanation.
> > > > > >> > > > > >
> > > > > >> > > > > > RIck's commit as below
> > > > > >> > > > > > https://www.mail-archive.com/u-boot at lists.denx.de/msg305880.html
> > > > > >> > > > >
> > > > > >> > > > > When we run U-Boot in S-mode the BBL runs from 0x80000000 so this
> > > > > >> > > > > two lines corrupts BBL instructions.
> > > > > >> > > > >
> > > > > >> > > > > If this is important for some board then please have it around #ifdef.
> > > > > >> > > > >
> > > > > >> > > >
> > > > > >> > > > Hi Anup
> > > > > >> > > >
> > > > > >> > > > In the discussion as below :
> > > > > >> > > > https://www.mail-archive.com/u-boot at lists.denx.de/msg305880.html
> > > > > >> > > >
> > > > > >> > > > I try to solve this issue with the aptch
> > > > > >> > > > [PATCH] riscv: ax25-ae350: Pass dtb address to u-boot with a1 register
> > > > > >> > > > diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S
> > > > > >> > > > -       li      t0, CONFIG_SYS_SDRAM_BASE
> > > > > >> > > > -       SREG    a2, 0(t0)
> > > > > >> > > >
> > > > > >> > > > diff --git a/board/AndesTech/ax25-ae350/ax25-ae350.c
> > > > > >> > > > b/board/AndesTech/ax25-ae350/ax25-ae350.c
> > > > > >> > > >  void *board_fdt_blob_setup(void)
> > > > > >> > > >  {
> > > > > >> > > > -       void **ptr = (void *)CONFIG_SYS_SDRAM_BASE;
> > > > > >> > > > +       void **ptr = (void *)&prior_stage_fdt_address;
> > > > > >> > > >
> > > > > >> > > > in the previous pull request.
> > > > > >> > > >
> > > > > >> > > > But Bin do not agree with that I use prior_stage_fdt_address in
> > > > > >> > > > board_fdt_blob_setup( )
> > > > > >> > > > I try to explain why I use it like that way.
> > > > > >> > > > Then Bin have not any reply in the following mail.
> > > > > >> > > > Finally I decide to drop this patch in the next pull request.
> > > > > >> > > >
> > > > > >> > > > Hi Bin
> > > > > >> > > >
> > > > > >> > > > How do you think about I recovery this patch to fix this issue ?
> > > > > >> > >
> > > > > >> > > Actually, previous booting stage can pass location of FDT stored in flash
> > > > > >> > > to U-Boot. U-Boot requires FDT at a DRAM location which it can modify
> > > > > >> > > in-place before passing on to Linux kernel so we should relocate the FDT
> > > > > >> > > passed by previous booting stage to some board specific DRAM location.
> > > > > >> > >
> > > > > >> > > My suggestion is as follows:
> > > > > >> > >
> > > > > >> > > Instead of SDRAM_BASE, we can have new board specific config
> > > > > >> > > CONFIG_RISCV_PRIOR_FDT_BASE
> > > > > >> > >
> > > > > >> > > If CONFIG_RISCV_PRIOR_FDT_BASE is defined/selected by
> > > > > >> > > config then in start.S copy-over the FDT from location pointed by
> > > > > >> > > "a1" register to location pointed by CONFIG_RISCV_PRIOR_FDT_BASE.
> > > > > >> >
> > > > > >>
> > > > > >> Hi Anup
> > > > > >>
> > > > > >> It can not achieve dtb delivery at runtime.
> > > > > >
> > > > > >
> > > > > > Can you elaborate why?
> > > > > >
> > > > >
> > > > > CONFIG_RISCV_PRIOR_FDT_BASE is determined at compile time.
> > > > > I am wondering how it can be modified at run time ?
> > > >
> > > > I think you miss-understood my suggestion. I did not meant changing
> > > > CONFIG_RISCV_PRIOR_FDT_BASE at runtime.
> > > >
> > > > >
> > > > >
> > > > > > I am sure we can always relocate FDT passed by previous stage to some safer location and use it from there.
> > > > >
> > > > > My original patch also can achieve that dtb passed by a1 and relocated and boot.
> > > >
> > > > Great !!!
> > > >
> > > > Why not update your original patch to relocate FDT and use FDT from
> > > > safer location?
> > > >
> > >
> > > Good question.
> > > Above can see the question I ask for Bin :
> > > How do you think about I recovery this patch to fix this issue ?
> > >
> > > Before you talking about CONFIG_RISCV_PRIOR_FDT_BASE !
> >
> > Can you explain why you need CONFIG_OF_BOARD ?
> > Why can you not use CONFIG_OF_PRIOR_STAGE ?
> >
>
> You can find the discussion as below:
> https://patchwork.ozlabs.org/patch/988884/
>

If I understand correctly, so far we have two scenarios to support:

1. U-Boot is booting directly from reset vector from ROM. This
canonical way to support DT is via CONFIG_OF_EMBED or
CONFIG_OF_SEPARATE. In such configuration, there is no any previous
bootloader so the concept of CONFIG_OF_PRIOR_STAGE does not apply.

2. U-Boot is booting from previous bootloader, and DT is passed in a1
register. Such configuration we can use CONFIG_OF_PRIOR_STAGE.

Regards,
Bin