From: Jeff Dike 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 Signed-off-by: Andrew Morton --- arch/um/drivers/chan_kern.c | 3 +++ arch/um/drivers/chan_user.c | 11 ++++++++++- arch/um/drivers/line.c | 9 +++------ 3 files changed, 16 insertions(+), 7 deletions(-) diff -puN arch/um/drivers/chan_kern.c~uml-fix-console-writing-bugs arch/um/drivers/chan_kern.c --- a/arch/um/drivers/chan_kern.c~uml-fix-console-writing-bugs +++ a/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)) diff -puN arch/um/drivers/chan_user.c~uml-fix-console-writing-bugs arch/um/drivers/chan_user.c --- a/arch/um/drivers/chan_user.c~uml-fix-console-writing-bugs +++ a/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, diff -puN arch/um/drivers/line.c~uml-fix-console-writing-bugs arch/um/drivers/line.c --- a/arch/um/drivers/line.c~uml-fix-console-writing-bugs +++ a/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) { _