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>

3 files changed, 16 insertions, 7 deletions

diff --git a/arch/um/drivers/chan_kern.c b/arch/um/drivers/chan_kern.c
index c09dbdfa298a..db3082b4da46 100644
--- a/arch/um/drivers/chan_kern.c
+++ b/arch/um/drivers/chan_kern.c
@@ -291,6 +291,9 @@ int write_chan(struct list_head *chans, const char *buf, int len,
         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))
diff --git a/arch/um/drivers/chan_user.c b/arch/um/drivers/chan_user.c
index 77557e233f58..d29e56d8ced4 100644
--- a/arch/um/drivers/chan_user.c
+++ b/arch/um/drivers/chan_user.c
@@ -38,7 +38,16 @@ int generic_read(int fd, char *c_out, void *unused)
 
 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,
diff --git a/arch/um/drivers/line.c b/arch/um/drivers/line.c
index 3e0b68e297f2..b4538dfb4820 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) {