avr32: Add simple paging support

Use the MMU hardware to set up 1:1 mappings between physical and virtual
addresses. This allows us to bypass the cache when accessing the flash
without having to do any physical-to-virtual address mapping in the CFI
driver.

The virtual memory mappings are defined at compile time through a sorted
array of virtual memory range objects. When a TLB miss exception
happens, the exception handler does a binary search through the array
until it finds a matching entry and loads it into the TLB. The u-boot
image itself is covered by a fixed TLB entry which is never replaced.

This makes the 'saveenv' command work again on ATNGW100 and other boards
using the CFI driver, hopefully without breaking any rules.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>

1 files changed, 66 insertions, 0 deletions

diff --git a/arch/avr32/include/asm/arch-at32ap700x/mmu.h b/arch/avr32/include/asm/arch-at32ap700x/mmu.h
new file mode 100644
index 0000000000..fcd9a05609
--- /dev/null
+++ b/arch/avr32/include/asm/arch-at32ap700x/mmu.h
@@ -0,0 +1,66 @@
+/*
+ * In order to deal with the hardcoded u-boot requirement that virtual
+ * addresses are always mapped 1:1 with physical addresses, we implement
+ * a small virtual memory manager so that we can use the MMU hardware in
+ * order to get the caching properties right.
+ *
+ * A few pages (or possibly just one) are locked in the TLB permanently
+ * in order to avoid recursive TLB misses, but most pages are faulted in
+ * on demand.
+ */
+#ifndef __ASM_ARCH_MMU_H
+#define __ASM_ARCH_MMU_H
+
+#include <asm/sysreg.h>
+
+#define PAGE_SHIFT	20
+#define PAGE_SIZE	(1UL << PAGE_SHIFT)
+#define PAGE_ADDR_MASK	(~(PAGE_SIZE - 1))
+
+#define MMU_VMR_CACHE_NONE						\
+	(SYSREG_BF(AP, 3) | SYSREG_BF(SZ, 3) | SYSREG_BIT(TLBELO_D))
+#define MMU_VMR_CACHE_WBUF						\
+	(MMU_VMR_CACHE_NONE | SYSREG_BIT(B))
+#define MMU_VMR_CACHE_WRTHRU						\
+	(MMU_VMR_CACHE_NONE | SYSREG_BIT(TLBELO_C) | SYSREG_BIT(W))
+#define MMU_VMR_CACHE_WRBACK						\
+	(MMU_VMR_CACHE_WBUF | SYSREG_BIT(TLBELO_C))
+
+/*
+ * This structure is used in our "page table". Instead of the usual
+ * x86-inspired radix tree, we let each entry cover an arbitrary-sized
+ * virtual address range and store them in a binary search tree. This is
+ * somewhat slower, but should use significantly less RAM, and we
+ * shouldn't get many TLB misses when using 1 MB pages anyway.
+ *
+ * With 1 MB pages, we need 12 bits to store the page number. In
+ * addition, we stick an Invalid bit in the high bit of virt_pgno (if
+ * set, it cannot possibly match any faulting page), and all the bits
+ * that need to be written to TLBELO in phys_pgno.
+ */
+struct mmu_vm_range {
+	uint16_t	virt_pgno;
+	uint16_t	nr_pages;
+	uint32_t	phys;
+};
+
+/*
+ * An array of mmu_vm_range objects describing all pageable addresses.
+ * The array is sorted by virt_pgno so that the TLB miss exception
+ * handler can do a binary search to find the correct entry.
+ */
+extern struct mmu_vm_range mmu_vmr_table[];
+
+/*
+ * Initialize the MMU. This will set up a fixed TLB entry for the static
+ * u-boot image at dest_addr and enable paging.
+ */
+void mmu_init_r(unsigned long dest_addr);
+
+/*
+ * Handle a TLB miss exception. This function is called directly from
+ * the exception vector table written in assembly.
+ */
+int mmu_handle_tlb_miss(void);
+
+#endif /* __ASM_ARCH_MMU_H */