Re: [RFC 23/23] ARM: keystone: add switch over to high physical address range
On 7/24/2012 10:39 AM, Arnd Bergmann wrote:
On Tuesday 24 July 2012, Cyril Chemparathy wrote:Keystone platforms have their physical memory mapped at an address outside the 32-bit physical range. A Keystone machine with 16G of RAM would find its memory at 0x0800000000 - 0x0bffffffff. For boot purposes, the interconnect supports a limited alias of some of this memory within the 32-bit addressable space (0x80000000 - 0xffffffff). This aliasing is implemented in hardware, and is not intended to be used much beyond boot. For instance, DMA coherence does not work when running out of this aliased address space. Therefore, we've taken the approach of booting out of the low physical address range, and subsequently we switch over to the high range once we're safely inside machine specific territory. This patch implements this switch over mechanism, which involves rewiring the TTBRs and page tables to point to the new physical address space. Signed-off-by: Vitaly Andrianov <vitalya@xxxxxx> Signed-off-by: Cyril Chemparathy <cyril@xxxxxx>I think this needs some more explanations. Why is not not possible to use this the larger area from the start when we first enable paging?
By enable paging, I assume you refer to the head.S init. For this the boot code needs to get the "real physical address" from somewhere instead of having to deduce it from the program counter. We could do this by parsing DTB in the decompressor, and passing in a 64-bit physmem pointer into the kernel startup code.
We'd considered this approach (at least briefly), but then balked at (a) having to change the entry conditions into head.S code, and (b) baking in dependencies on the decompressor.
Also, the code does not really look platform specific, so I could imagine that if you need it, other similar platforms will need the same thing, and it should be put into common code and enabled all the time when using LPAE.
Absolutely agreed. Vitaly and I have been trying to work it out this way, and we hope to have something more common in the next version of this series.
-- Thanks - Cyril _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
[Linux ARM (vger)] [Linux ARM MSM] [Linux Omap] [Linux Arm] [Linux Tegra] [Fedora ARM] [eCos] [Linux Fastboot] [Gcc Help] [Git] [DCCP] [IETF Announce] [Security] [PDAs] [Linux] [Linux MIPS] [Yosemite Campsites] [Photos]