x86, pat: Allow ISA memory range uncacheable mapping requests

Max Vozeler reported:
>  Bug 13877 -  bogl-term broken with CONFIG_X86_PAT=y, works with =n
>
>  strace of bogl-term:
>  814   mmap2(NULL, 65536, PROT_READ|PROT_WRITE, MAP_SHARED, 4, 0)
>				 = -1 EAGAIN (Resource temporarily unavailable)
>  814   write(2, "bogl: mmaping /dev/fb0: Resource temporarily unavailable\n",
>	       57) = 57

PAT code maps the ISA memory range as WB in the PAT attribute, so that
fixed range MTRR registers define the actual memory type (UC/WC/WT etc).

But the upper level is_new_memtype_allowed() API checks are failing,
as the request here is for UC and the return tracked type is WB (Tracked type is
WB as MTRR type for this legacy range potentially will be different for each
4k page).

Fix is_new_memtype_allowed() by always succeeding the ISA address range
checks, as the null PAT (WB) and def MTRR fixed range register settings
satisfy the memory type needs of the applications that map the ISA address
range.

Reported-and-Tested-by: Max Vozeler <xam@debian.org>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/include/asm/pgtable.h

@@ -2,6 +2,7 @@
 #define _ASM_X86_PGTABLE_H
 
 #include <asm/page.h>
+#include <asm/e820.h>
 
 #include <asm/pgtable_types.h>
 
@@ -269,9 +270,16 @@ static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
 
 #define canon_pgprot(p) __pgprot(massage_pgprot(p))
 
-static inline int is_new_memtype_allowed(unsigned long flags,
-						unsigned long new_flags)
+static inline int is_new_memtype_allowed(u64 paddr, unsigned long size,
+					 unsigned long flags,
+					 unsigned long new_flags)
 {
+	/*
+	 * PAT type is always WB for ISA. So no need to check.
+	 */
+	if (is_ISA_range(paddr, paddr + size - 1))
+		return 1;
+
 	/*
 	 * Certain new memtypes are not allowed with certain
 	 * requested memtype:

arch/x86/mm/pat.c

@@ -623,7 +623,8 @@ static int reserve_pfn_range(u64 paddr, unsigned long size, pgprot_t *vma_prot,
 		return ret;
 
 	if (flags != want_flags) {
-		if (strict_prot || !is_new_memtype_allowed(want_flags, flags)) {
+		if (strict_prot ||
+		    !is_new_memtype_allowed(paddr, size, want_flags, flags)) {
 			free_memtype(paddr, paddr + size);
 			printk(KERN_ERR "%s:%d map pfn expected mapping type %s"
 				" for %Lx-%Lx, got %s\n",