Blackfin: audit UART for all known anomalies

There is no code change here, just new comments, but this keeps me from
having to do another audit from scratch in the future.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

cpu/blackfin/serial.c

@@ -24,6 +24,19 @@
  * Licensed under the GPL-2 or later.
  */
 
+/* Anomaly notes:
+ *  05000086 - we don't support autobaud
+ *  05000099 - we only use DR bit, so losing others is not a problem
+ *  05000100 - we don't use the UART_IIR register
+ *  05000215 - we poll the uart (no dma/interrupts)
+ *  05000225 - no workaround possible, but this shouldnt cause errors ...
+ *  05000230 - we tweak the baud rate calculation slightly
+ *  05000231 - we always use 1 stop bit
+ *  05000309 - we always enable the uart before we modify it in anyway
+ *  05000350 - we always enable the uart regardless of boot mode
+ *  05000363 - we don't support break signals, so don't generate one
+ */
+
 #include <common.h>
 #include <watchdog.h>
 #include <asm/blackfin.h>
@@ -43,7 +56,9 @@ uint16_t cached_rbr[256];
 size_t cache_count;
 
 /* The LSR is read-to-clear on some parts, so we have to make sure status
- * bits aren't inadvertently lost when doing various tests.
+ * bits aren't inadvertently lost when doing various tests.  This also
+ * works around anomaly 05000099 at the same time by keeping a cumulative
+ * tally of all the status bits.
  */
 static uint16_t uart_lsr_save;
 static uint16_t uart_lsr_read(void)
@@ -59,6 +74,10 @@ static void uart_lsr_clear(void)
 	*pUART_LSR |= -1;
 }
 #else
+/* When debugging is disabled, we only care about the DR bit, so if other
+ * bits get set/cleared, we don't really care since we don't read them
+ * anyways (and thus anomaly 05000099 is irrelevant).
+ */
 static inline uint16_t uart_lsr_read(void) { return *pUART_LSR; }
 static inline void uart_lsr_clear(void) { *pUART_LSR = -1; }
 #endif

cpu/blackfin/serial.h

@@ -146,7 +146,7 @@ static inline void serial_early_init(void)
 	/* handle portmux crap on different Blackfins */
 	serial_do_portmux();
 
-	/* Enable UART */
+	/* always enable UART -- avoids anomalies 05000309 and 05000350 */
 	*pUART_GCTL = UCEN;
 
 	/* Set LCR to Word Lengh 8-bit word select */