x86: mtrr: add CONFIG_CACHE_ROM support

The CONFIG_CACHE_ROM support in the MTRR code allocates an MTRR
specifically for setting up write-protect cachine of the ROM. It is
assumed that CONFIG_ROM_SIZE is the size of the ROM and the whole
area should be cached just under 4GiB. If enabled, the MTRR code
will allocate but not enable rom caching. It is up to the callers
of the MTRR code to explicitly enable (and disable afterwards) through
the use of 2 new functions:
- x86_mtrr_enable_rom_caching()
- x86_mtrr_disable_rom_caching()

Additionally, the CACHE_ROM option is exposed to the config menu so
that it is not just selected by the chipset or board. The reasoning
is that through a multitude of options CACHE_ROM may not be appropriate
for enabling.

Change-Id: I4483df850f442bdcef969ffeaf7608ed70b88085
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/2918
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/src/include/cpu/x86/mtrr.h b/src/include/cpu/x86/mtrr.h
index 618a93c44e..bff736d296 100644
--- a/src/include/cpu/x86/mtrr.h
+++ b/src/include/cpu/x86/mtrr.h
@@ -52,6 +52,10 @@
  *    of the nature of the global MTRR enable flag. Therefore, all direct
  *    or indirect callers of enable_fixed_mtrr() should ensure that the
  *    variable MTRR MSRs do not contain bad ranges.
+ * 3. If CONFIG_CACHE_ROM is selected an MTRR is allocated for enabling
+ *    the caching of the ROM. However, it is set to uncacheable (UC). It
+ *    is the responsiblity of the caller to enable it by calling
+ *    x86_mtrr_enable_rom_caching().
  */
 void x86_setup_mtrrs(void);
 /*
@@ -67,6 +71,18 @@ void x86_setup_fixed_mtrrs(void);
 /* Set up fixed MTRRs but do not enable them. */
 void x86_setup_fixed_mtrrs_no_enable(void);
 int x86_mtrr_check(void);
+/* ROM caching can be used after variable MTRRs are set up. Beware that
+ * enabling CONFIG_CACHE_ROM will eat through quite a few MTRRs based on
+ * one's IO hole size and WRCOMB resources. Be sure to check the console
+ * log when enabling CONFIG_CACHE_ROM or adding WRCOMB resources. */
+#if CONFIG_CACHE_ROM
+void x86_mtrr_enable_rom_caching(void);
+void x86_mtrr_disable_rom_caching(void);
+#else
+static inline void x86_mtrr_enable_rom_caching(void) {}
+static inline void x86_mtrr_disable_rom_caching(void) {}
+#endif /* CONFIG_CACHE_ROM */
+
 #endif
 
 #if !defined(CONFIG_RAMTOP)