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.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 6 deletions

diff --git a/arch/um/drivers/line.c b/arch/um/drivers/line.c
index 3e0b68e297f..b4538dfb482 100644
--- a/arch/um/drivers/line.c
+++ b/arch/um/drivers/line.c
@@ -216,18 +216,15 @@ int line_write(struct tty_struct *tty, const unsigned char *buf, int len)
 {
 	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) {