powerpc/8xx: Map 32Mb of RAM at init.

At the time being, initial MMU setup allows 24 Mbytes
of DATA and 8 Mbytes of code.

Some debug setup like CONFIG_KASAN generate huge
kernels with text size over the 8M limit and data over the
24 Mbytes limit.

Here is an 8xx kernel compiled with CONFIG_KASAN_INLINE for
one of my boards:

[root@po16846vm linux-powerpc]# size -x vmlinux
   text	   data	    bss	    dec	    hex	filename
0x111019c	0x41b0d4	0x490de0	26984528	19bc050	vmlinux

This patch maps up to 32 Mbytes code based on _einittext symbol
and allows 32 Mbytes of memory instead of 24.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/8xx_mmu.c

@@ -112,6 +112,9 @@ unsigned long __init mmu_mapin_ram(unsigned long top)
 			mmu_patch_cmp_limit(&patch__itlbmiss_linmem_top, 0);
 	} else {
 		mapped = top & ~(LARGE_PAGE_SIZE_8M - 1);
+		if (!IS_ENABLED(CONFIG_PIN_TLB_TEXT))
+			mmu_patch_cmp_limit(&patch__itlbmiss_linmem_top,
+					    _ALIGN(__pa(_einittext), 8 << 20));
 	}
 
 	mmu_patch_cmp_limit(&patch__dtlbmiss_linmem_top, mapped);
@@ -140,8 +143,8 @@ void __init setup_initial_memory_limit(phys_addr_t first_memblock_base,
 	 */
 	BUG_ON(first_memblock_base != 0);
 
-	/* 8xx can only access 24MB at the moment */
-	memblock_set_current_limit(min_t(u64, first_memblock_size, 0x01800000));
+	/* 8xx can only access 32MB at the moment */
+	memblock_set_current_limit(min_t(u64, first_memblock_size, 0x02000000));
 }
 
 /*