[efi] Add EFI entropy source

Originally-implemented-by: Jarrod Johnson <jbjohnso@us.ibm.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/efi/efi_entropy.h

@@ -0,0 +1,35 @@
+#ifndef _IPXE_EFI_ENTROPY_H
+#define _IPXE_EFI_ENTROPY_H
+
+/** @file
+ *
+ * EFI entropy source
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <stdint.h>
+
+#ifdef ENTROPY_EFI
+#define ENTROPY_PREFIX_efi
+#else
+#define ENTROPY_PREFIX_efi __efi_
+#endif
+
+/**
+ * min-entropy per sample
+ *
+ * @ret min_entropy	min-entropy of each sample
+ */
+static inline __always_inline double
+ENTROPY_INLINE ( efi, min_entropy_per_sample ) ( void ) {
+
+	/* We use essentially the same mechanism as for the BIOS
+	 * RTC-based entropy source, and so assume the same
+	 * min-entropy per sample.
+	 */
+	return 1.3;
+}
+
+#endif /* _IPXE_EFI_ENTROPY_H */

src/include/ipxe/entropy.h

@@ -54,6 +54,7 @@ typedef uint8_t entropy_sample_t;
 
 /* Include all architecture-independent entropy API headers */
 #include <ipxe/null_entropy.h>
+#include <ipxe/efi/efi_entropy.h>
 #include <ipxe/linux/linux_entropy.h>
 
 /* Include all architecture-dependent entropy API headers */