ChangeSet 1.2261, 2005/03/31 08:36:46-08:00, [EMAIL PROTECTED]
[PATCH] uml: Fix the console stuttering
I spent far too much of the weekend tracking this sucker down through
the guts
of the tty code. The problem turns out to be that drivers/char/n_tty.c
has a
write_chan that does buffering and retransmitting data, and
arch/um/drivers/chan_kern.c ALSO has a write_chan that buffers and
retransmits
data, and the first calls the second but the second doesn't always
return
correct status information for the -EAGAIN case.
When they get confused, both of them try to buffer and retransmit data,
hence
the stuttering.
The first fix is that if chan_kern's write_chan gets an -EAGAIN, it
should NOT
gratuitously change that to a 0 before returning. I don't know why
that code
is in there, but deleting those two lines makes 90% the stuttering go
away.
But not quite all of it.
The second half of the fix is arch/um/drivers/line.c has a buffer_data()
function that adds data to the buffer, tries to flush the buffer out to
disk,
gets -EAGAIN, and then returns -EAGAIN even though it successfully
buffered
all the data it was sent. So the upper layer resubmits the last chunk
of data
it sent when the console unblocks, even though the lower layer buffered
it and
sent it on by that point.
With this patch, I can't get the UML console to stutter anymore by
suspending
the process it's writing to. (Add tee to the mix and you can still
make it
hang by suspending its xterm for a second or two, but I think that tee
is
hanging, not UML. Hangs with RHEL4 tee, but not busybox tee...)
Signed-off-by: Rob Landley <[EMAIL PROTECTED]>
Acked-by: Jeff Dike <[EMAIL PROTECTED]>
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <[EMAIL PROTECTED]>
Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>
chan_kern.c | 5 +----
line.c | 2 +-
2 files changed, 2 insertions(+), 5 deletions(-)
diff -Nru a/arch/um/drivers/chan_kern.c b/arch/um/drivers/chan_kern.c
--- a/arch/um/drivers/chan_kern.c 2005-03-31 10:17:49 -08:00
+++ b/arch/um/drivers/chan_kern.c 2005-03-31 10:17:49 -08:00
@@ -248,11 +248,8 @@
n = chan->ops->write(chan->fd, buf, len, chan->data);
if (chan->primary) {
ret = n;
- if ((ret == -EAGAIN) || ((ret >= 0) && (ret < len))){
+ if ((ret == -EAGAIN) || ((ret >= 0) && (ret < len)))
reactivate_fd(chan->fd, write_irq);
- if (ret == -EAGAIN)
- ret = 0;
- }
}
}
return(ret);
diff -Nru a/arch/um/drivers/line.c b/arch/um/drivers/line.c
--- a/arch/um/drivers/line.c 2005-03-31 10:17:49 -08:00
+++ b/arch/um/drivers/line.c 2005-03-31 10:17:49 -08:00
@@ -128,7 +128,7 @@
ret = buffer_data(line, buf, len);
err = flush_buffer(line);
local_irq_restore(flags);
- if(err <= 0)
+ if(err <= 0 && (err != -EAGAIN || !ret))
ret = err;
}
else {
-
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