linux-kernel - Re: [PATCH] RISC-V: Add an Image header that boot loader can parse.

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Message-ID: <20190501203248.oadcldefusc2ighv@excalibur.cnev.de>
Date:   Wed, 1 May 2019 22:32:48 +0200
From:   Karsten Merker <merker@...ian.org>
To:     Anup Patel <anup@...infault.org>,
        Mark Rutland <mark.rutland@....com>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        Palmer Dabbelt <palmer@...ive.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "zong@...estech.com" <zong@...estech.com>,
        Atish Patra <atish.patra@....com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>
Cc:     Karsten Merker <merker@...ian.org>
Subject: Re: [PATCH] RISC-V: Add an Image header that boot loader can parse.

On Wed, May 01, 2019 at 09:54:43PM +0200, Karsten Merker wrote:
> On Wed, May 01, 2019 at 10:41:52PM +0530, Anup Patel wrote:
> > On Wed, May 1, 2019 at 10:30 PM Mark Rutland <mark.rutland@....com> wrote:
> > > On Mon, Apr 29, 2019 at 10:42:40PM -0700, Atish Patra wrote:
> > > > On 4/29/19 4:40 PM, Palmer Dabbelt wrote:
> > > > > On Tue, 23 Apr 2019 16:25:06 PDT (-0700), atish.patra@....com wrote:

> Probably I'm missing something obvious, but I cannot completely
> follow you here. My understanding is as follows:
[...]
> If the first byte in a PE/COFF header has to be an ASCII "M",
> that is 01001101 in binary.  RISC-V is little-endian and the last
> two bits of the lowest-value byte define the type of instruction. 
> According to the chapter "Base Instruction-Length Encoding" in
> the RISC-V ISA spec everything except 11 as the lowest bits
> denotes a compressed instruction and if I have puzzeled together
> the the various instruction bits correctly, ASCII "MZ" would be
> excuted as a compressed load immediate to x9/s1, wouldn't it?

Sorry, I have misinterpreted a bitfield in the spec, it's indeed
a compressed load immediate to x20/s4.

Regards,
Karsten
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