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SubjectRe: [PATCH 0/2] Early printk support for virtio console devices.
Date
Peter Maydell <peter.maydell@linaro.org> writes:
> On 1 May 2013 03:07, Rusty Russell <rusty@rustcorp.com.au> wrote:
>> An emergency output is a reasonable idea, and this is a reasonable
>> implementation. The question is practical: will it be used? Because we
>> don't implement reasonable ideas which aren't going to be used.
>
> If you think it fits reasonably into the virtio spec (ie doesn't
> implement things at the wrong level of the transport/backend
> abstraction) then we can implement it in QEMU, and I think it
> makes more sense to do this than to throw in a random extra
> serial port.
>
> To be actually useful we need to also specify something in
> the device tree to say "here is where you will find your
> emergency output and what it is".

Hmm, I'm not sure that's true. It looks like it needs:

1) An enhancment to the vdev->set_config callback to pass through (at
least) an offset, probably offset and length.

2) An emerg_write() function ptr which can be called at any time, set by
virtio_console.c's class_init:

static void emerg_write(VirtIOSerialPort *port, char c)
{
VirtConsole *vcon = DO_UPCAST(VirtConsole, port, port);

if (vcon->chr)
qemu_chr_fe_write(vcon->chr, &c, 1);
}

3) A routine to find an emerg-write-capable console in
virtio_serial_bus.c (or just assume port 0?):

static VirtIOSerialPort *find_emerg_write_port(VirtIOSerial *vser)
{
VirtIOSerialPort *port;

QTAILQ_FOREACH(port, &vser->ports, next) {
VirtIOSerialPortClass *vsc;
vsc = VIRTIO_SERIAL_PORT_GET_CLASS(port);

if (vsc->emerg_write) {
return port;
}
}
return NULL;
}

4) set_config in virtio_serial_bus.c to notice emergency writes:

if (offset == offsetof(struct virtio_console_config, emerg_w) {
VirtIOSerial *vser;
vser = DO_UPCAST(VirtIOSerial, vdev, vdev);
VirtIOSerialPort *port;

port = find_emerg_write_port(vser);
if (port) {
vsc->emerg_write(port, config.emerg_w);
}
}

Amit might have more clue... Amit?

Thanks,
Rusty.

From: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Subject: virtio: console: Add early writeonly register to config space

This patch adds a emerg_wr register (writeonly) in config space of virtio console device which can be used for debugging.

Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Signed-off-by: Anup Patel <anup.patel@linaro.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
include/uapi/linux/virtio_console.h | 3 +++
1 file changed, 3 insertions(+)

diff --git a/include/uapi/linux/virtio_console.h b/include/uapi/linux/virtio_console.h
index ee13ab6..586678d 100644
--- a/include/uapi/linux/virtio_console.h
+++ b/include/uapi/linux/virtio_console.h
@@ -38,6 +38,7 @@
/* Feature bits */
#define VIRTIO_CONSOLE_F_SIZE 0 /* Does host provide console size? */
#define VIRTIO_CONSOLE_F_MULTIPORT 1 /* Does host provide multiple ports? */
+#define VIRTIO_CONSOLE_F_EMERG_WRITE 2 /* Does host support emergency write? */

#define VIRTIO_CONSOLE_BAD_ID (~(u32)0)

@@ -48,6 +49,8 @@ struct virtio_console_config {
__u16 rows;
/* max. number of ports this device can hold */
__u32 max_nr_ports;
+ /* emergency write register */
+ __u32 emerg_wr;
} __attribute__((packed));

/*

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