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bcm64: handle 8GB of physical memory for raspberry pi4

widen and move the KMAP window to a new address so we can
handle the 8GB of physical memory of the new raspberry pi4.

the new memory map on pi4 uses the following 4 banks:

0x000000000	0x03e600000
0x040000000	0x0fc000000 <- soc.dramsize (only < 4GB)
0x100000000	0x180000000
0x180000000 0x200000000

--- a/sys/src/9/bcm64/mem.h

+++ b/sys/src/9/bcm64/mem.h

@@ -39,8 +39,10 @@

 #define STACKALIGN(sp)	((sp) & ~7)		/* bug: assure with alloc */

 #define TRAPFRAMESIZE	(38*8)

-#define KSEG0		(0xFFFFFFFE00000000ULL)

-#define KMAP		(0xFFFFFFFE00000000ULL)

+#define KSEG0		(0xFFFFFFFC00000000ULL)

+

+#define KMAP		(0xFFFFFFFC00000000ULL)

+#define KMAPEND		(0xFFFFFFFF00000000ULL)

 #define FRAMEBUFFER	(0xFFFFFFFFA0000000ULL|PTEWT)

--- a/sys/src/9/bcm64/mmu.c

+++ b/sys/src/9/bcm64/mmu.c

@@ -132,12 +132,13 @@

 	return nil;

 }

-/* KMAP maps all of ram (up to 4GB) */

 static void*

 kmapaddr(uintptr pa)

 {

 	if(pa < (uintptr)-KZERO)

 		return (void*)(pa + KZERO);

+	if(pa >= KMAPEND-KMAP)

+		panic("kmapaddr: pa=%#p pc=%#p", pa, getcallerpc(&pa));

 	return (void*)(pa + KMAP);

 }

@@ -275,12 +276,16 @@

 	pa = PGROUND((uintptr)end)-KZERO;

 	for(i=0; i<nelem(conf.mem); i++){

-		if(conf.mem[i].limit <= conf.mem[i].base

-		|| conf.mem[i].base >= PHYSDRAM + soc.dramsize){

-			conf.mem[i].base = conf.mem[i].limit = 0;

+		if(conf.mem[i].limit >= KMAPEND-KMAP)

+			conf.mem[i].limit = KMAPEND-KMAP;

+

+		if(conf.mem[i].limit <= conf.mem[i].base){

+			conf.mem[i].limit = conf.mem[i].base = 0;

 			continue;

 		}

-		if(conf.mem[i].limit > PHYSDRAM + soc.dramsize)

+

+		if(conf.mem[i].base < PHYSDRAM + soc.dramsize

+		&& conf.mem[i].limit > PHYSDRAM + soc.dramsize)

 			conf.mem[i].limit = PHYSDRAM + soc.dramsize;

 		/* take kernel out of allocatable space */