[v3, 3/7] powerpc: enable the relocatable support for the fsl booke 32bit kernel
scottwood at freescale.com
Thu Jan 9 08:46:04 EST 2014
On Wed, 2014-01-08 at 10:42 +0800, Kevin Hao wrote:
> On Tue, Jan 07, 2014 at 05:46:04PM -0600, Scott Wood wrote:
> > On Sat, 2014-01-04 at 14:34 +0800, Kevin Hao wrote:
> > > > I'm having a hard time following the logic here. What is PAGE_OFFSET -
> > > > offset supposed to be? Why would we map anything belowe PAGE_OFFSET?
> > >
> > > No, we don't map the address below PAGE_OFFSET.
> > > memstart_addr is the physical start address of RAM.
> > > start is the kernel running physical address aligned with 64M.
> > >
> > > offset = memstart_addr - start
> > >
> > > So if memstart_addr < start, the offset is negative. The PAGE_OFFSET - offset
> > > is the virtual start address we should use for the init 64M map. It's above
> > > the PAGE_OFFSET instead of below.
> > Oh. I think it'd be more readable to do "offset = start -
> > memstart_addr" and add offset instead of subtracting it.
> Yes, I agree. The reason that I use "offset = memstart_addr - start" is that
> it seems "memstart_addr" is always greater than "start" when we are booting
> a kdump kernel with a kernel option like "crashkernel=64M at 80M". :-)
...so there is a situation where you map below PAGE_OFFSET. :-)
> > Also, offset should be phys_addr_t -- even if you don't expect to
> > support offsets greater than 4G on 32-bit, it's semantically the right
> > type to use. Plus, "int" would break if this code were ever used with
> > 64-bit.
> I thought about using phy_addr_t for the "offset" originally but gave it up
> for the following reasons:
> * It will not be greater than 4G.
> * We have to use the ugly #ifdef CONFIG_PHYS_64BIT in restore_to_as0().
> * Need more registers for arguments for restore_to_as0().
> Of course you can change it to phys_addr_t if you prefer.
I'd at least like to make it "long".
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