25985edced
Fixes generated by 'codespell' and manually reviewed. Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
245 lines
11 KiB
XML
245 lines
11 KiB
XML
Memory management for CRIS/MMU
|
|
------------------------------
|
|
HISTORY:
|
|
|
|
$Log: README.mm,v $
|
|
Revision 1.1 2001/12/17 13:59:27 bjornw
|
|
Initial revision
|
|
|
|
Revision 1.1 2000/07/10 16:25:21 bjornw
|
|
Initial revision
|
|
|
|
Revision 1.4 2000/01/17 02:31:59 bjornw
|
|
Added discussion of paging and VM.
|
|
|
|
Revision 1.3 1999/12/03 16:43:23 hp
|
|
Blurb about that the 3.5G-limitation is not a MMU limitation
|
|
|
|
Revision 1.2 1999/12/03 16:04:21 hp
|
|
Picky comment about not mapping the first page
|
|
|
|
Revision 1.1 1999/12/03 15:41:30 bjornw
|
|
First version of CRIS/MMU memory layout specification.
|
|
|
|
|
|
|
|
|
|
|
|
------------------------------
|
|
|
|
See the ETRAX-NG HSDD for reference.
|
|
|
|
We use the page-size of 8 kbytes, as opposed to the i386 page-size of 4 kbytes.
|
|
|
|
The MMU can, apart from the normal mapping of pages, also do a top-level
|
|
segmentation of the kernel memory space. We use this feature to avoid having
|
|
to use page-tables to map the physical memory into the kernel's address
|
|
space. We also use it to keep the user-mode virtual mapping in the same
|
|
map during kernel-mode, so that the kernel easily can access the corresponding
|
|
user-mode process' data.
|
|
|
|
As a comparison, the Linux/i386 2.0 puts the kernel and physical RAM at
|
|
address 0, overlapping with the user-mode virtual space, so that descriptor
|
|
registers are needed for each memory access to specify which MMU space to
|
|
map through. That changed in 2.2, putting the kernel/physical RAM at
|
|
0xc0000000, to co-exist with the user-mode mapping. We will do something
|
|
quite similar, but with the additional complexity of having to map the
|
|
internal chip I/O registers and the flash memory area (including SRAM
|
|
and peripherial chip-selets).
|
|
|
|
The kernel-mode segmentation map:
|
|
|
|
------------------------ ------------------------
|
|
FFFFFFFF| | => cached | |
|
|
| kernel seg_f | flash | |
|
|
F0000000|______________________| | |
|
|
EFFFFFFF| | => uncached | |
|
|
| kernel seg_e | flash | |
|
|
E0000000|______________________| | DRAM |
|
|
DFFFFFFF| | paged to any | Un-cached |
|
|
| kernel seg_d | =======> | |
|
|
D0000000|______________________| | |
|
|
CFFFFFFF| | | |
|
|
| kernel seg_c |==\ | |
|
|
C0000000|______________________| \ |______________________|
|
|
BFFFFFFF| | uncached | |
|
|
| kernel seg_b |=====\=========>| Registers |
|
|
B0000000|______________________| \c |______________________|
|
|
AFFFFFFF| | \a | |
|
|
| | \c | FLASH/SRAM/Peripheral|
|
|
| | \h |______________________|
|
|
| | \e | |
|
|
| | \d | |
|
|
| kernel seg_0 - seg_a | \==>| DRAM |
|
|
| | | Cached |
|
|
| | paged to any | |
|
|
| | =======> |______________________|
|
|
| | | |
|
|
| | | Illegal |
|
|
| | |______________________|
|
|
| | | |
|
|
| | | FLASH/SRAM/Peripheral|
|
|
00000000|______________________| |______________________|
|
|
|
|
In user-mode it looks the same except that only the space 0-AFFFFFFF is
|
|
available. Therefore, in this model, the virtual address space per process
|
|
is limited to 0xb0000000 bytes (minus 8192 bytes, since the first page,
|
|
0..8191, is never mapped, in order to trap NULL references).
|
|
|
|
It also means that the total physical RAM that can be mapped is 256 MB
|
|
(kseg_c above). More RAM can be mapped by choosing a different segmentation
|
|
and shrinking the user-mode memory space.
|
|
|
|
The MMU can map all 4 GB in user mode, but doing that would mean that a
|
|
few extra instructions would be needed for each access to user mode
|
|
memory.
|
|
|
|
The kernel needs access to both cached and uncached flash. Uncached is
|
|
necessary because of the special write/erase sequences. Also, the
|
|
peripherial chip-selects are decoded from that region.
|
|
|
|
The kernel also needs its own virtual memory space. That is kseg_d. It
|
|
is used by the vmalloc() kernel function to allocate virtual contiguous
|
|
chunks of memory not possible using the normal kmalloc physical RAM
|
|
allocator.
|
|
|
|
The setting of the actual MMU control registers to use this layout would
|
|
be something like this:
|
|
|
|
R_MMU_KSEG = ( ( seg_f, seg ) | // Flash cached
|
|
( seg_e, seg ) | // Flash uncached
|
|
( seg_d, page ) | // kernel vmalloc area
|
|
( seg_c, seg ) | // kernel linear segment
|
|
( seg_b, seg ) | // kernel linear segment
|
|
( seg_a, page ) |
|
|
( seg_9, page ) |
|
|
( seg_8, page ) |
|
|
( seg_7, page ) |
|
|
( seg_6, page ) |
|
|
( seg_5, page ) |
|
|
( seg_4, page ) |
|
|
( seg_3, page ) |
|
|
( seg_2, page ) |
|
|
( seg_1, page ) |
|
|
( seg_0, page ) );
|
|
|
|
R_MMU_KBASE_HI = ( ( base_f, 0x0 ) | // flash/sram/periph cached
|
|
( base_e, 0x8 ) | // flash/sram/periph uncached
|
|
( base_d, 0x0 ) | // don't care
|
|
( base_c, 0x4 ) | // physical RAM cached area
|
|
( base_b, 0xb ) | // uncached on-chip registers
|
|
( base_a, 0x0 ) | // don't care
|
|
( base_9, 0x0 ) | // don't care
|
|
( base_8, 0x0 ) ); // don't care
|
|
|
|
R_MMU_KBASE_LO = ( ( base_7, 0x0 ) | // don't care
|
|
( base_6, 0x0 ) | // don't care
|
|
( base_5, 0x0 ) | // don't care
|
|
( base_4, 0x0 ) | // don't care
|
|
( base_3, 0x0 ) | // don't care
|
|
( base_2, 0x0 ) | // don't care
|
|
( base_1, 0x0 ) | // don't care
|
|
( base_0, 0x0 ) ); // don't care
|
|
|
|
NOTE: while setting up the MMU, we run in a non-mapped mode in the DRAM (0x40
|
|
segment) and need to setup the seg_4 to a unity mapping, so that we don't get
|
|
a fault before we have had time to jump into the real kernel segment (0xc0). This
|
|
is done in head.S temporarily, but fixed by the kernel later in paging_init.
|
|
|
|
|
|
Paging - PTE's, PMD's and PGD's
|
|
-------------------------------
|
|
|
|
[ References: asm/pgtable.h, asm/page.h, asm/mmu.h ]
|
|
|
|
The paging mechanism uses virtual addresses to split a process memory-space into
|
|
pages, a page being the smallest unit that can be freely remapped in memory. On
|
|
Linux/CRIS, a page is 8192 bytes (for technical reasons not equal to 4096 as in
|
|
most other 32-bit architectures). It would be inefficient to let a virtual memory
|
|
mapping be controlled by a long table of page mappings, so it is broken down into
|
|
a 2-level structure with a Page Directory containing pointers to Page Tables which
|
|
each have maps of up to 2048 pages (8192 / sizeof(void *)). Linux can actually
|
|
handle 3-level structures as well, with a Page Middle Directory in between, but
|
|
in many cases, this is folded into a two-level structure by excluding the Middle
|
|
Directory.
|
|
|
|
We'll take a look at how an address is translated while we discuss how it's handled
|
|
in the Linux kernel.
|
|
|
|
The example address is 0xd004000c; in binary this is:
|
|
|
|
31 23 15 7 0
|
|
11010000 00000100 00000000 00001100
|
|
|
|
|______| |__________||____________|
|
|
PGD PTE page offset
|
|
|
|
Given the top-level Page Directory, the offset in that directory is calculated
|
|
using the upper 8 bits:
|
|
|
|
static inline pgd_t * pgd_offset(struct mm_struct * mm, unsigned long address)
|
|
{
|
|
return mm->pgd + (address >> PGDIR_SHIFT);
|
|
}
|
|
|
|
PGDIR_SHIFT is the log2 of the amount of memory an entry in the PGD can map; in our
|
|
case it is 24, corresponding to 16 MB. This means that each entry in the PGD
|
|
corresponds to 16 MB of virtual memory.
|
|
|
|
The pgd_t from our example will therefore be the 208'th (0xd0) entry in mm->pgd.
|
|
|
|
Since the Middle Directory does not exist, it is a unity mapping:
|
|
|
|
static inline pmd_t * pmd_offset(pgd_t * dir, unsigned long address)
|
|
{
|
|
return (pmd_t *) dir;
|
|
}
|
|
|
|
The Page Table provides the final lookup by using bits 13 to 23 as index:
|
|
|
|
static inline pte_t * pte_offset(pmd_t * dir, unsigned long address)
|
|
{
|
|
return (pte_t *) pmd_page(*dir) + ((address >> PAGE_SHIFT) &
|
|
(PTRS_PER_PTE - 1));
|
|
}
|
|
|
|
PAGE_SHIFT is the log2 of the size of a page; 13 in our case. PTRS_PER_PTE is
|
|
the number of pointers that fit in a Page Table and is used to mask off the
|
|
PGD-part of the address.
|
|
|
|
The so-far unused bits 0 to 12 are used to index inside a page linearily.
|
|
|
|
The VM system
|
|
-------------
|
|
|
|
The kernels own page-directory is the swapper_pg_dir, cleared in paging_init,
|
|
and contains the kernels virtual mappings (the kernel itself is not paged - it
|
|
is mapped linearily using kseg_c as described above). Architectures without
|
|
kernel segments like the i386, need to setup swapper_pg_dir directly in head.S
|
|
to map the kernel itself. swapper_pg_dir is pointed to by init_mm.pgd as the
|
|
init-task's PGD.
|
|
|
|
To see what support functions are used to setup a page-table, let's look at the
|
|
kernel's internal paged memory system, vmalloc/vfree.
|
|
|
|
void * vmalloc(unsigned long size)
|
|
|
|
The vmalloc-system keeps a paged segment in kernel-space at 0xd0000000. What
|
|
happens first is that a virtual address chunk is allocated to the request using
|
|
get_vm_area(size). After that, physical RAM pages are allocated and put into
|
|
the kernel's page-table using alloc_area_pages(addr, size).
|
|
|
|
static int alloc_area_pages(unsigned long address, unsigned long size)
|
|
|
|
First the PGD entry is found using init_mm.pgd. This is passed to
|
|
alloc_area_pmd (remember the 3->2 folding). It uses pte_alloc_kernel to
|
|
check if the PGD entry points anywhere - if not, a page table page is
|
|
allocated and the PGD entry updated. Then the alloc_area_pte function is
|
|
used just like alloc_area_pmd to check which page table entry is desired,
|
|
and a physical page is allocated and the table entry updated. All of this
|
|
is repeated at the top-level until the entire address range specified has
|
|
been mapped.
|
|
|
|
|
|
|