linux-kernel - Re: [PATCH v4 0/4] MIPS: Modify mem= and memmap= parameter

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Date:   Sat, 5 Mar 2022 22:09:34 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     "Maciej W. Rozycki" <macro@...am.me.uk>
Cc:     Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        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 0/4] MIPS: Modify mem= and memmap= parameter

On Sat, Mar 05, 2022 at 07:21:15PM +0000, Maciej W. Rozycki wrote:
> On Sat, 5 Mar 2022, Mike Rapoport wrote:
> 
> > > >  For example I have an x86 system that Linux does not how to interrogate
> > > > for RAM beyond 64MiB, so I do use `memmap=128M@0' (for legacy reasons the
> > > > x86 platform has a special exception to always exclude area between 640K
> > > > and 1M from being used even if not explicitly specified, but we do not
> > > > have a need for such legacy such legacy concerns with the MIPS port).  I
> > > > consider it an interim measure however until the kernel has been fixed.
> > > > 
> > > >   Maciej
> > > > 
> > > 
> > > Hi Mike, Thomas and Maciej,
> > > 
> > > Thank you very much for your feedbacks and discussions.
> > > 
> > > To be frank, I think mem= and memmap= are used for debugging and testing
> > > in most cases, the intention of this patchset is to refactor the related
> > > code to make them work well on mips.
> > 
> > mem= works fine on mips and there is no need to change it.
> > 
> > If you must supply complex memory layout on the command line, consider
> > implementing support for memmap=exact and multiple memmap= parameters on
> > the kernel command line, like x86 does.
> 
>  There's nothing to implement as the MIPS port has supported arbitrary 
> memory maps since Dec 11th, 2000; that's almost 22 years now.  C.f.: 
> <https://lore.kernel.org/linux-mips/Pine.GSO.3.96.1000814133957.7256S-100000@delta.ds2.pg.gda.pl/>, 
> <https://git.kernel.org/pub/scm/linux/kernel/git/ralf/linux.git/commit/?id=97b7ae4257ef>.

You are right, and providing mem=X@Y for each contiguous memory range
should work even after 22 years.
I missed the fact that mem= can be repeated several times.
 
>  Sadly commit a09fc446fb6d ("[MIPS] setup.c: use early_param() for early 
> command line parsing") removed last pieces of inline documentation; I 
> don't know why things like that get approved, but neither I can take 
> responsibility.

This is sad indeed, but we still can update the kernel-parameters.txt with
a MIPS paragraph.
 
>   Maciej

-- 
Sincerely yours,
Mike.