Question About Linux Memory Mapping

[Drasko DRASKOVIC]

Thanks for your answers.

I can see now that "mem" parameter is parsed in
arch/arm/kernel/setup.c, as Nicolas underlined. However, in my case
early_mem() is never called during the Linux boot.

Does anybody have an idea why ?

BR,
Drasko


On Mon, Feb 21, 2011 at 6:27 AM, hong zhang <henryzhang62 at yahoo.com> wrote:
> Drasko,
>
> Command line can assign all physical memory or partial memory to kernel using one or few mem=. For example, if you have 1 GBytes memory with starting port at 0x80000000, and want to reserve 512M anywhere between 0x80000000 and 0xc0000000 saying at 0x90000000. Then command line should be
> mem=256M at 0x80000000 mem=256M at 0xb0000000. At this format, kernel should not touch memory between 0x9000000 and 0xb0000000 and the memory can be used by other OS or firmware.
>
> This is my understanding.
>
> ---henry
>
> --- On Sat, 2/19/11, Marek Vasut <marek.vasut at gmail.com> wrote:
>
>> From: Marek Vasut <marek.vasut at gmail.com>
>> Subject: Re: Question About Linux Memory Mapping
>> To: linux-arm at lists.infradead.org
>> Cc: "Drasko DRASKOVIC" <drasko.draskovic at gmail.com>, linux-arm-kernel at lists.infradead.org
>> Date: Saturday, February 19, 2011, 2:22 PM
>> On Friday 18 February 2011 21:56:28
>> Drasko DRASKOVIC wrote:
>> > Hi all,
>> > in the article Booting ARM Linux :
>> > http://www.simtec.co.uk/products/SWLINUX/files/booting_article.html
>> > I can see that mem map is passed via ATAG_MEM.
>> However, in the same
>> > article it is mentioned that this information can also
>> be passed via
>> > kernel command line paramters,
>> mem=<size>[KM][,@<phys_offset>].
>> >
>> > However, this does not seem to be true, as "mem"
>> command line
>> > parameter seems to be formated like this : mem= n[KMG]
>> (i.e. no
>> > offset), regarding to this reference :
>> > http://oreilly.com/linux/excerpts/9780596100797/kernel-boot-command-line-pa
>> > rameter-reference.html. Seems like memmap should be
>> used instead.
>> >
>> > I tried passing the parameters like memmap=
>> n[KMG]@start[KMG] but that
>> > had no effect at all - still the same amount of System
>> Ram was read
>> > from ATAGS and presented in the system via
>> /proc/iomem.
>> >
>> > What I needed it to reserve 1MB region for one FIFO at
>> the end of RAM
>> > (or somewhere else)
>> > and protect it from the kernel. I tried passing
>> memmap=
>> > n[KMG]$start[KMG], but that did not worn neither.
>>
>> What are you exactly trying to achieve ? btw. if you really
>> need to make a hole
>> in RAM, you should reserve a bootmem node maybe?
>> >
>> > So my questions are following :
>> > 1) Why commandlines are ignored and ATAGS are given
>> priority ?
>> > 2) What is the most elegant way to protect one region
>> in RAM :
>> >  a) By giving less memory with ATAGS_MEM and thus
>> making protected
>> > region invisible to Linux, lying to it that RAM is
>> smaller
>> >  b) By changing somehow linker script
>> >  c) By changing some configuration variables
>> (which ?)
>> >
>> > Thanks for the answers and best regards,
>> > Drasko
>> >
>> > _______________________________________________
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>> > linux-arm at lists.infradead.org
>> > http://lists.infradead.org/mailman/listinfo/linux-arm
>>
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