x86: Add a function to set variable MTRRs

Normally U-Boot handles MTRRs through an add/commit process which
overwrites all MTRRs. But in very early boot it is not desirable to clear
the existing MTRRs since they may be in use and it can cause a hang.

Add a new mtrr_set_next_var() function which sets up the next available
MTRR to the required region.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: pass 'type' to set_var_mtrr() in mtrr_set_next_var()]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/cpu/mtrr.c

@@ -112,3 +112,41 @@ int mtrr_add_request(int type, uint64_t start, uint64_t size)
 
 	return 0;
 }
+
+static int get_var_mtrr_count(void)
+{
+	return msr_read(MSR_MTRR_CAP_MSR).lo & MSR_MTRR_CAP_VCNT;
+}
+
+static int get_free_var_mtrr(void)
+{
+	struct msr_t maskm;
+	int vcnt;
+	int i;
+
+	vcnt = get_var_mtrr_count();
+
+	/* Identify the first var mtrr which is not valid */
+	for (i = 0; i < vcnt; i++) {
+		maskm = msr_read(MTRR_PHYS_MASK_MSR(i));
+		if ((maskm.lo & MTRR_PHYS_MASK_VALID) == 0)
+			return i;
+	}
+
+	/* No free var mtrr */
+	return -ENOSPC;
+}
+
+int mtrr_set_next_var(uint type, uint64_t start, uint64_t size)
+{
+	int mtrr;
+
+	mtrr = get_free_var_mtrr();
+	if (mtrr < 0)
+		return mtrr;
+
+	set_var_mtrr(mtrr, type, start, size);
+	debug("MTRR %x: start=%x, size=%x\n", mtrr, (uint)start, (uint)size);
+
+	return 0;
+}

arch/x86/include/asm/mtrr.h

@@ -117,6 +117,18 @@ int mtrr_add_request(int type, uint64_t start, uint64_t size);
  */
 int mtrr_commit(bool do_caches);
 
+/**
+ * mtrr_set_next_var() - set up a variable MTRR
+ *
+ * This finds the first free variable MTRR and sets to the given area
+ *
+ * @type:	Requested type (MTRR_TYPE_)
+ * @start:	Start address
+ * @size:	Size
+ * @return 0 on success, -ENOSPC if there are no more MTRRs
+ */
+int mtrr_set_next_var(uint type, uint64_t base, uint64_t size);
+
 #endif
 
 #if ((CONFIG_XIP_ROM_SIZE & (CONFIG_XIP_ROM_SIZE - 1)) != 0)