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SubjectRe: [PATCH 0/2] Early printk support for virtio console devices.
From
Hi Rusty,

On 6 May 2013 10:41, Rusty Russell <rusty@rustcorp.com.au> wrote:
> 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));
>
> /*

I will send above patch with V2 tagged with the addition of
documentation change and also separate arm64 early printk
implementation for the same.

Thanks,
Pranav


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