Hi folks,
On Tue, Jul 05, 2011 at 04:22:43PM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2011-07-05 at 04:17 +0000, Tabi Timur-B04825 wrote:
> > On Mon, Jul 4, 2011 at 9:24 AM, Hendrik Brueckner
> > <brueck...@linux.vnet.ibm.com> wrote:
> >
> > I started that thread. After much soul searching, we came to the
> > conclusion that HVC is not compatible with hypervisors that return
> > BUSY on writes.
>
> That is a fun conclusion considering that hvc has been written for the
> pseries hypervisor which ... can return BUSY on writes :-)
>
> We just need to fix HVC properly.
So I took initiative and looked again into this issue. Below you can
find a patch that is based on Ben's -EAGAIN idea. The hvc console layer
takes care of retrying depending on the backend's return code.
However, the main issue is that from a backend perspective, there is no
difference between tty and console output. Because consoles, especially
the preferred console, behave different than a simple tty. Blocked write
to a preferred console can stop the system.
So with the patch below, the backend can now indirectly control the way
console output is handled for it. I still have to think if this solution
is ok or if it is better to introduce a new callback to console output only
(and might provide a default implemenatation similar to the patch below).
NOTE: I did not yet test this patch but will do.. I just want to share it
early to get feedback from you.
-->8---------------------------------------------------------------------
Currently, the hvc_console_print() function drops console output if the
hvc backend's put_chars() returns 0. This patch changes this behavior
to allow a retry through returning -EAGAIN.
This change also affects the hvc_push() function. Both functions are
changed to handle -EAGAIN and to retry the put_chars() operation.
If a hvc backend returns -EAGAIN, the retry handling differs:
- hvc_console_print() spins to write the complete console output.
- hvc_push() behaves the same way as for returning 0.
Now hvc backends can indirectly control the way how console output is
handled through the hvc console layer.
Signed-off-by: Hendrik Brueckner <brueck...@linux.vnet.ibm.com>
---
drivers/tty/hvc/hvc_console.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
--- a/drivers/tty/hvc/hvc_console.c
+++ b/drivers/tty/hvc/hvc_console.c
@@ -163,8 +163,10 @@ static void hvc_console_print(struct con
} else {
r = cons_ops[index]->put_chars(vtermnos[index], c, i);
if (r <= 0) {
- /* throw away chars on error */
- i = 0;
+ /* throw away characters on error
+ * but spin in case of -EAGAIN */
+ if (r != -EAGAIN)
+ i = 0;
} else if (r > 0) {
i -= r;
if (i > 0)
@@ -448,7 +450,7 @@ static int hvc_push(struct hvc_struct *h
n = hp->ops->put_chars(hp->vtermno, hp->outbuf, hp->n_outbuf);
if (n <= 0) {
- if (n == 0) {
+ if (n == 0 || n == -EAGAIN) {
hp->do_wakeup = 1;
return 0;
}
_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@lists.ozlabs.org
https://lists.ozlabs.org/listinfo/linuxppc-dev
