From: Grant Erickson <gerickson@nuovations•com>
To: "linuxppc-dev@ozlabs•org" <linuxppc-dev@ozlabs•org>
Subject: Reserving Early RAM Regions and Porting Denx's CONFIG_LOGBUFFER to arch/powerpc
Date: Tue, 28 Oct 2008 15:41:11 -0700 [thread overview]
Message-ID: <C52CE317.12884%gerickson@nuovations.com> (raw)
I am working on attempting to migrate the Denx CONFIG_LOGBUFFER feature from
arch/ppc:
http://git.denx.de/?p=linux-2.6-denx.git;a=blob_plain;f=arch/ppc/mm/init.c;h
b=3df65660bbfa769b10b141351b0ea10427b0b709
http://git.denx.de/?p=linux-2.6-denx.git;a=blob_plain;f=kernel/printk.c;hb=3
df65660bbfa769b10b141351b0ea10427b0b709
In the historical implementation, basically, u-boot (if so configured)
allocates 16 KiB + 4 KiB of memory at the end of physical RAM to store log
messages that are compatible with the Linux kernel's printk. When Linux (if
so configured with CONFIG_LOGBUFFER) starts up, it redirects the normal
log_buf to this "external log buffer" and appends any existing printk
messages to those already pushed by u-boot. Future printk messages are then
sent off to this new "external log buffer".
What is the preferred method for hiding/reserving chunks of memory such as
this during early boot? The device tree? The 'mem=' parameter? Adding an
lmb_reserve() in prom.c:early_reserve_mem()? Twiddling things auto-magically
such as the historical implementation? Some combination thereof?
The arch/ppc historical implementation simply twiddled total_memory and
total_lowmem and then ioremap'd the 20 KiB off the end of memory.
However, such an approach in arch/powerpc falters because an attempt to
prematurely reduce total_lowmem results in mapin_ram(), via mmu_mapin_ram(),
only covering part of the total, say 128 MiB or 256 MiB, RAM with 16 MiB and
4 MiB large pages, leaving a residual, of say 4 MiB, to cover with smaller 4
KiB map_page() pages.
However, when map_page() runs, it eventually calls lmb_alloc_base() which
returns a page in that 4 MiB "tail" unmapped region. This then explodes due
to a page fault when clear_page() is called on the page in unmapped RAM at
the dcbz instruction in clear_pages().
Regards,
Grant
next reply other threads:[~2008-10-28 22:48 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-10-28 22:41 Grant Erickson [this message]
2008-10-28 23:01 ` Reserving Early RAM Regions and Porting Denx's CONFIG_LOGBUFFER to arch/powerpc Wolfgang Denk
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