[timer] Rewrite the 8254 Programmable Interval Timer support

The 8254 timer code (used to implement udelay()) has an unknown
provenance.  Rewrite this code to avoid potential licensing
uncertainty.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/include/ipxe/pit8254.h

@@ -0,0 +1,81 @@
+#ifndef _IPXE_PIT8254_H
+#define _IPXE_PIT8254_H
+
+/** @file
+ *
+ * 8254 Programmable Interval Timer
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+/** IRQ0 channel */
+#define PIT8254_CH_IRQ0 0
+
+/** PC speaker channel */
+#define PIT8254_CH_SPKR 2
+
+/** Timer frequency (1.193182MHz) */
+#define PIT8254_HZ 1193182UL
+
+/** Data port */
+#define PIT8254_DATA(channel) ( 0x40 + (channel) )
+
+/** Mode/command register */
+#define PIT8254_CMD 0x43
+
+/** Select channel */
+#define PIT8254_CMD_CHANNEL(channel) ( (channel) << 6 )
+
+/** Access modes */
+#define PIT8254_CMD_ACCESS_LATCH 0x00	/**< Latch count value command */
+#define PIT8254_CMD_ACCESS_LO	0x10	/**< Low byte only */
+#define PIT8254_CMD_ACCESS_HI	0x20	/**< High byte only */
+#define PIT8254_CMD_ACCESS_LOHI	0x30	/**< Low-byte, high-byte pair */
+
+/* Operating modes */
+#define PIT8254_CMD_OP_TERMINAL	0x00	/**< Interrupt on terminal count */
+#define PIT8254_CMD_OP_ONESHOT	0x02	/**< Hardware re-triggerable one-shot */
+#define PIT8254_CMD_OP_RATE	0x04	/**< Rate generator */
+#define PIT8254_CMD_OP_SQUARE	0x06	/**< Square wave generator */
+#define PIT8254_CMD_OP_SWSTROBE	0x08	/**< Software triggered strobe */
+#define PIT8254_CMD_OP_HWSTROBE	0x0a	/**< Hardware triggered strobe */
+#define PIT8254_CMD_OP_RATE2	0x0c	/**< Rate generator (duplicate) */
+#define PIT8254_CMD_OP_SQUARE2	0x0e	/**< Square wave generator (duplicate)*/
+
+/** Binary mode */
+#define PIT8254_CMD_BINARY 0x00
+
+/** BCD mode */
+#define PIT8254_CMD_BCD 0x01
+
+/** PC speaker control register */
+#define PIT8254_SPKR 0x61
+
+/** PC speaker channel gate */
+#define PIT8254_SPKR_GATE 0x01
+
+/** PC speaker enabled */
+#define PIT8254_SPKR_ENABLE 0x02
+
+/** PC speaker channel output */
+#define PIT8254_SPKR_OUT 0x20
+
+extern void pit8254_speaker_delay ( unsigned int ticks );
+
+/**
+ * Delay for a fixed number of microseconds
+ *
+ * @v usecs		Number of microseconds for which to delay
+ */
+static inline __attribute__ (( always_inline )) void
+pit8254_udelay ( unsigned long usecs ) {
+
+	/* Delays are invariably compile-time constants; force the
+	 * multiplication and division to take place at compilation
+	 * time rather than runtime.
+	 */
+	pit8254_speaker_delay ( ( usecs * PIT8254_HZ ) / 1000000 );
+}
+
+#endif /* _IPXE_PIT8254_H */