UML - Fix console writing bugs

The previous console cleanup patch switched generic_read and generic_write from calling os_{read,write}_file to calling read and write directly. Because the calling convention is different, they now need to get any error from errno rather than the return value. I did this for generic_read, but forgot about generic_write.

While chasing some output corruption, I noticed that line_write was unnecessarily calling flush_buffer, and deleted it. I don't understand why, but the corruption disappeared. This is unneeded because there already is a perfectly good mechanism for finding out when the host output device has some room to write data - there is an interrupt that comes in when writes can happen again. line_write calling flush_buffer seemed to just be an attempt to opportunistically get some data out to the host.

I also made write_chan short-circuit calling into the host-level code for zero-length writes. Calling libc write with a length of zero conflated write not being able to write anything with asking it not to write anything. Better to just cut it off as soon as possible.

--

 arch/um/drivers/chan_kern.c |    3 +++
 arch/um/drivers/chan_user.c |   11 ++++++++++-
 arch/um/drivers/line.c      |    5 +----
 3 files changed, 14 insertions(+), 5 deletions(-)

Index: linux-2.6.21-mm/arch/um/drivers/chan_kern.c
===================================================================
@@ -291,6 +291,9 @@ int write_chan(struct list_head *chans, 
 	struct chan *chan = NULL;
 	int n, ret = 0;
 
+	if (len == 0)
+		return 0;
+
 	list_for_each(ele, chans) {
 		chan = list_entry(ele, struct chan, list);
 		if (!chan->output || (chan->ops->write == NULL))

Index: linux-2.6.21-mm/arch/um/drivers/chan_user.c
===================================================================
@@ -38,7 +38,16 @@ int generic_read(int fd, char *c_out, vo
 
 int generic_write(int fd, const char *buf, int n, void *unused)
 {
-	return write(fd, buf, n);
+	int err;
+
+	err = write(fd, buf, n);
+	if (err > 0)
+		return err;
+	else if (errno == EAGAIN)
+		return 0;
+	else if (err == 0)
+		return -EIO;
+	return -errno;
 }
 
 int generic_window_size(int fd, void *unused, unsigned short *rows_out,

Index: linux-2.6.21-mm/arch/um/drivers/line.c
===================================================================
@@ -216,18 +216,15 @@ int line_write(struct tty_struct *tty, c
 {
 	struct line *line = tty->driver_data;
 	unsigned long flags;
-	int n, err, ret = 0;
+	int n, ret = 0;
 
 	if(tty->stopped)
 		return 0;
 
 	spin_lock_irqsave(&line->lock, flags);
-	if (line->head != line->tail) {
+	if (line->head != line->tail)
 		ret = buffer_data(line, buf, len);
-		err = flush_buffer(line);
-		if (err <= 0 && (err != -EAGAIN || !ret))
-			ret = err;
-	} else {
+	else {
 		n = write_chan(&line->chan_list, buf, len,
 			       line->driver->write_irq);
 		if (n < 0) {

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