mtrr: Define a function for obtaining free var mtrr

Instead of hard-coding var mtrr numbers in code, use this function to
identify the first available variable mtrr. If no such mtrr is
available, the function will return -1.

Change-Id: I2a1e02cdb45c0ab7e30609641977471eaa2431fd
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/14115
Tested-by: build bot (Jenkins)
Reviewed-by: Andrey Petrov <andrey.petrov@intel.com>

src/cpu/x86/mtrr/earlymtrr.c

@@ -2,6 +2,30 @@
 #include <cpu/x86/mtrr.h>
 #include <cpu/x86/msr.h>
 
+/* Get first available variable MTRR.
+ * Returns var# if available, else returns -1.
+ */
+int get_free_var_mtrr(void)
+{
+	msr_t msr, maskm;
+	int vcnt;
+	int i;
+
+	/* Read MTRRCap and get vcnt - variable memory type ranges. */
+	msr = rdmsr(MTRR_CAP_MSR);
+	vcnt = msr.lo & 0xff;
+
+	/* Identify the first var mtrr which is not valid. */
+	for (i = 0; i < vcnt; i++) {
+		maskm = rdmsr(MTRR_PHYS_MASK(i));
+		if ((maskm.lo & MTRR_PHYS_MASK_VALID) == 0)
+			return i;
+	}
+
+	/* No free var mtrr. */
+	return -1;
+}
+
 #ifdef __ROMCC__
 static
 #endif

src/include/cpu/x86/mtrr.h

@@ -81,6 +81,7 @@ void x86_mtrr_check(void);
 
 #if !defined(__ASSEMBLER__) && defined(__PRE_RAM__) && !defined(__ROMCC__)
 void set_var_mtrr(unsigned reg, unsigned base, unsigned size, unsigned type);
+int get_free_var_mtrr(void);
 #endif
 
 /* Align up to next power of 2, suitable for ROMCC and assembler too.