spi: bitbang bugfix in message setup

Bugfix to spi_bitbang infrastructure: make sure to always set transfer
parameters on the first pass through the message's per-transfer loop.
This can matter with drivers that replace the per-word or per-buffer
transfer primitives, on busses with multiple SPI devices.

Previously, this could have started messages using the settings left after
previous messages.  The problem was observed when a high speed chip
(m25p80 type flash) was running very slowly because a low speed device
(avr8 microcontroller) had previously used the bus.  Similar faults could
have driven the low speed device too fast, or used an unexpected word
size.
Acked-by: NSteven A. Falco <sfalco@harris.com>
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/spi/spi_bitbang.c

@@ -258,6 +258,11 @@ static void bitbang_work(struct work_struct *work)
 	struct spi_bitbang	*bitbang =
 		container_of(work, struct spi_bitbang, work);
 	unsigned long		flags;
+	int			do_setup = -1;
+	int			(*setup_transfer)(struct spi_device *,
+					struct spi_transfer *);
+
+	setup_transfer = bitbang->setup_transfer;
 
 	spin_lock_irqsave(&bitbang->lock, flags);
 	bitbang->busy = 1;
@@ -269,8 +274,6 @@ static void bitbang_work(struct work_struct *work)
 		unsigned		tmp;
 		unsigned		cs_change;
 		int			status;
-		int			(*setup_transfer)(struct spi_device *,
-						struct spi_transfer *);
 
 		m = container_of(bitbang->queue.next, struct spi_message,
 				queue);
@@ -287,19 +290,19 @@ static void bitbang_work(struct work_struct *work)
 		tmp = 0;
 		cs_change = 1;
 		status = 0;
-		setup_transfer = NULL;
 
 		list_for_each_entry (t, &m->transfers, transfer_list) {
 
-			/* override or restore speed and wordsize */
-			if (t->speed_hz || t->bits_per_word) {
-				setup_transfer = bitbang->setup_transfer;
+			/* override speed or wordsize? */
+			if (t->speed_hz || t->bits_per_word)
+				do_setup = 1;
+
+			/* init (-1) or override (1) transfer params */
+			if (do_setup != 0) {
 				if (!setup_transfer) {
 					status = -ENOPROTOOPT;
 					break;
 				}
-			}
-			if (setup_transfer) {
 				status = setup_transfer(spi, t);
 				if (status < 0)
 					break;
@@ -363,9 +366,10 @@ static void bitbang_work(struct work_struct *work)
 		m->status = status;
 		m->complete(m->context);
 
-		/* restore speed and wordsize */
-		if (setup_transfer)
+		/* restore speed and wordsize if it was overridden */
+		if (do_setup == 1)
 			setup_transfer(spi, NULL);
+		do_setup = 0;
 
 		/* normally deactivate chipselect ... unless no error and
 		 * cs_change has hinted that the next message will probably