ioremap to a specific virtual address

[Nicolas Pitre]

On Sun, 1 Apr 2012, jonsmirl at gmail.com wrote:

> On Sun, Apr 1, 2012 at 3:46 PM, Arnd Bergmann <arnd at arndb.de> wrote:
> > On Sunday 01 April 2012, jonsmirl at gmail.com wrote:
> >> Looks good, way cleaner than my x86 desktop which is full of 8KB
> >> mappings. On my ARM I had about 50 regions before since I was mapping
> >> each device individually.
> >>
> >> root at OpenWrt:/proc# cat vmallocinfo
> >> 0xc4804000-0xc4810000   49152 cramfs_uncompress_init+0x24/0x60 pages=11 vmalloc
> >> 0xc4810000-0xc4853000  274432 jffs2_zlib_init+0x14/0xa4 pages=66 vmalloc
> >> 0xc4853000-0xc485f000   49152 jffs2_zlib_init+0x48/0xa4 pages=11 vmalloc
> >> 0xc485f000-0xc4886000  159744 ssd1289_probe+0x250/0x85c pages=38 vmalloc
> >> 0xf1300000-0xf1400000 1048576 iotable_init+0x0/0xb0 phys=13000000 ioremap
> >> 0xf1500000-0xf1600000 1048576 iotable_init+0x0/0xb0 phys=15000000 ioremap
> >> 0xf1600000-0xf1700000 1048576 iotable_init+0x0/0xb0 phys=16000000 ioremap
> >> 0xf1700000-0xf1800000 1048576 iotable_init+0x0/0xb0 phys=17000000 ioremap
> >> 0xf1800000-0xf1900000 1048576 iotable_init+0x0/0xb0 phys=18000000 ioremap
> >> 0xf1900000-0xf1a00000 1048576 iotable_init+0x0/0xb0 phys=19000000 ioremap
> >> 0xf2000000-0xf2100000 1048576 iotable_init+0x0/0xb0 phys=20000000 ioremap
> >> 0xf6000000-0xf6100000 1048576 iotable_init+0x0/0xb0 phys=60000000 ioremap
> >> 0xf7000000-0xf7100000 1048576 iotable_init+0x0/0xb0 phys=70000000 ioremap
> >> root at OpenWrt:/proc#
> >
> > Well, you loose the information about which devices specifically do the ioremap,
> > but that wasn't very reliable to start with. How about combining those that
> > are already consecutive areas, e.g. 0xf1300000-0xf1a00000? I don't see
> > any downsides of doing that, and it would save a few bytes here and there.
> 
>  0xf1300000-0xf1a00000 is the virtual address.  0x13000000-0x21000000
> is the physical address.  That's a span of 224MB.  How do the page
> table entries work on ARM? I thought we only got 1MB per entry.  So I
> need seven entries to span the range with the individual maps. I'd
> need 224 entries to cover the entire region. Or can we make make the
> page table entries any size we want?

You can't.  Arnd probably didn't notice that the physical range is not 
contiguous.  What you have is pretty optimal.

Of course you could avoid fragmenting the virtual space by packing all 
those mappings together, but I suspect there is a macro providing a 
correspondence between the virtual and physical addresses still in use.  
OTOH this is not worth bothering with unless you run out of vmalloc 
space.


Nicolas