linux-kernel - Re: [PATCH v4 1/4] MIPS: Refactor early_parse

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Message-ID: <alpine.DEB.2.21.2203041634040.47558@angie.orcam.me.uk>
Date:   Fri, 4 Mar 2022 17:11:44 +0000 (GMT)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>
cc:     Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Xuefeng Li <lixuefeng@...ngson.cn>, linux-mips@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/4] MIPS: Refactor early_parse_mem() to fix mem=
 parameter

On Fri, 4 Mar 2022, Thomas Bogendoerfer wrote:

> > > With this patch, when add "mem=3G" to the command-line, the
> > > kernel boots successfully, we can see the following messages:
> > 
> > unfortunately this patch would break platforms without memory detection,
> > which simply use mem=32M for memory configuration. Not sure how many
> > rely on this mechanism. If we can make sure nobody uses it, I'm fine
> > with your patch.
> 
> maybe we could add a CONFIG option, which will be selected by
> platforms, which don't need/want this usermem thing.

 FWIW I don't understand what the issue is here beyond that we have a bug 
that causes a system to hang when "mem=3G" is passed on the kernel command 
line.  That is assuming that system does have contiguous RAM available for 
the kernel to use from address 0 up to 3GiB; otherwise it's a user error 
to tell the kernel it has that memory available (I did get bitten by that 
myself too): garbage in, garbage out.

 I think having a CONFIG option automatically selected to disable the 
ability to give a memory map override would handicap people in debugging 
their systems or working around firmware bugs, so I would rather be 
against it.

  Maciej