[RFC v4 01/10] Add initial RISC-V architecture support

[Antony Pavlov]

On Mon, 2 Oct 2017 12:04:30 +0200
Daniel Schultz <d.schultz at phytec.de> wrote:

> Hi,
> 
> 
> On 09/30/2017 01:57 PM, Antony Pavlov wrote:
> > On Fri, 29 Sep 2017 14:07:09 +0200
> > Oleksij Rempel <linux at rempel-privat.de> wrote:
> >
> >> Hi,
> >>
> >> hm... mostly looks identical with existing arch
> > What do you mean when you say "existing arch"?
> >
> > ...
> >
> >> Am 29.09.2017 um 01:12 schrieb Antony Pavlov:
> > ...
> >>> diff --git a/arch/riscv/boot/start.S b/arch/riscv/boot/start.S
> >>> new file mode 100644
> >>> index 000000000..2fd00f63d
> >>> --- /dev/null
> >>> +++ b/arch/riscv/boot/start.S
> >>> @@ -0,0 +1,74 @@
> >>> +/*
> >>> + * Startup Code for MIPS CPU
> >>> + *
> >>> + * based on coreboot/src/arch/riscv/bootblock.S
> >>> + *
> >>> + * Copyright (C) 2016 Antony Pavlov <antonynpavlov at gmail.com>
> >>> + *
> >>> + * This file is part of barebox.
> >>> + * See file CREDITS for list of people who contributed to this project.
> >>> + *
> >>> + * This program is free software; you can redistribute it and/or modify
> >>> + * it under the terms of the GNU General Public License version 2
> >>> + * as published by the Free Software Foundation.
> >>> + *
> >>> + * This program is distributed in the hope that it will be useful,
> >>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> >>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> >>> + * GNU General Public License for more details.
> >>> + *
> >>> + */
> >>> +
> >>> +#include <asm-generic/memory_layout.h>
> >>> +
> >>> +	.text
> >>> +	.section ".text_entry"
> >>> +	.align 2
> >>> +
> >>> +.globl _start
> >>> +_start:
> >>> +	li sp, STACK_BASE + STACK_SIZE
> >>> +
> >>> +	# make room for HLS and initialize it
> >>> +	addi sp, sp, -64 /* MENTRY_FRAME_SIZE */
> >>> +
> >>> +	# poison the stack
> >>> +	li t1, STACK_BASE
> >>> +	li t0, 0xdeadbeef
> >>> +	sw t0, 0(t1)
> >>> +
> >>> +	# clear any pending interrupts
> >>> +	//csrwi mip, 0
> >> should be removed.
> > Actually not!
> >
> > I have imported this code from coreboot.
> >
> > I have commented this line because csrwi does not worked in some cases.
> >
> > But I have to make additional investigations on csrwi.
> >
> CSRRWI is part of the base integer instruction set and the machine mode 
> is mandatory. If there are troubles with this instruction, the core has 
> a faulty design. So executing this line should be okay even if there is 
> no interrupt controller.
> 

It's not a bug, it's a feature :)

At the moment I use picorv32 core.

Here is a quote from https://github.com/cliffordwolf/picorv32#custom-instructions-for-irq-handling

     The IRQ handling features in PicoRV32 do not follow the RISC-V Privileged ISA specification.
     Instead a small set of very simple custom instructions is used to implement IRQ handling with
     minimal hardware overhead.

I'm planning to make it possible to use another core instead of picorv32 in Eriso SoC,
so I can introduce something like SYS_HAS_PICORV32_CPU Kconfig macro.

> -- 
> Mit freundlichen Grüßen,
> With best regards,
>    Daniel Schultz
> 
> 
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-- 
Best regards,
  Antony Pavlov