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Date:   Tue, 31 Jan 2023 22:31:43 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     "Lad, Prabhakar" <prabhakar.csengg@gmail.com>,
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        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
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        Chris Paterson <Chris.Paterson2@renesas.com>
Subject: Re: [QUERY]: Block region to mmap
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On Mon, Jan 30, 2023 at 03:24:40PM +0000, Matthew Wilcox wrote:
> > Basically we are making use of the memory protection unit (MPU) so
> > that only M-mode is allowed to access this region and S/U modes are
> > blocked.
> 
> This sounds like RISC-V terminology.  I have no idea what M, S or U
> modes are (Supervisor and User, I'd guess for the last two?)


Yes, M = Machine, S = Supervisor, and U = User.
M omde is the absolutele worst idea of RISC-V and basically a mix
of microcode and super-SMM mode.

> Before we go too deeply into it, how much would it cost to buy all of
> these parts and feed them into a shredder?  I'm not entirely joking;
> if it's less than the software engineering time it'd take to develop
> and support this feature, we should do it.

The above suggests this is in no way an actual hardware problem, but the
stupid decision is done in the M-Mode firmware.  I think it is very
reasonable to simply not support the devices in Linux until the firmware
is fixed.