Re: [PATCH v2] hw/char/pl011: support backend hotswap

Hi Alexander,

On 11/8/26 10:14, Alexander Mikhalitsyn wrote:
> From: Alexander Mikhalitsyn <[email protected]>
> 
> Currently, when Incus issues "chardev-change" QMP command to change
> chardev backend from ringbuf to socket it receives an error (with aarch64 VM):
> "Chardev user does not support chardev hotswap" [1], [2]
> 
> Let's fix this by properly implementing BackendChangeHandler for pl011.
> 
> Please, note that we have to "replay" CHR_IOCTL_SERIAL_SET_BREAK, because
> if BRK bit was set before backend change (i.e. (s->lcr & LCR_BRK) is true),
> then after change we need to send break to a new backend too.
> 
> Link: https://discuss.linuxcontainers.org/t/unable-to-connect-to-vm-console-on-arm-architecture/23096/3 [1]
> Link: https://github.com/lxc/distrobuilder/issues/892 [2]
> Reported-by: Stéphane Graber <[email protected]>
> Reviewed-by: Alex Bennée <[email protected]>
> Signed-off-by: Alexander Mikhalitsyn <[email protected]>
> ---
> v2:
> 	- fixed a typo in commit author name
> 	  [ I did `git format-patch` and copied this patch from my Raspberry PI
> 	    dev/test machine and it turns out that I have a stupid typo in my
> 	    `git config get user.name` on that machine. ]
> 	- added RWB tag from Alex Bennée
> 	- adjusted a commit message
> ---
>   hw/char/pl011.c | 16 +++++++++++++++-
>   1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/char/pl011.c b/hw/char/pl011.c
> index cb12c3e224f..3622248ec0c 100644
> --- a/hw/char/pl011.c
> +++ b/hw/char/pl011.c
> @@ -660,12 +660,26 @@ static void pl011_init(Object *obj)
>       s->id = pl011_id_arm;
>   }
>   
> +static int pl011_be_change(void *opaque)
> +{
> +    PL011State *s = opaque;
> +    int break_enable = s->lcr & LCR_BRK;
> +
> +    qemu_chr_fe_set_handlers(&s->chr, pl011_can_receive, pl011_receive,
> +                             pl011_event, pl011_be_change, s, NULL, true);
> +
> +    qemu_chr_fe_ioctl(&s->chr, CHR_IOCTL_SERIAL_SET_BREAK,
> +                      &break_enable);

Could we factor the common code used in pl011_write() in a common
helper?

> +
> +    return 0;
> +}
> +
>   static void pl011_realize(DeviceState *dev, Error **errp)
>   {
>       PL011State *s = PL011(dev);
>   
>       qemu_chr_fe_set_handlers(&s->chr, pl011_can_receive, pl011_receive,
> -                             pl011_event, NULL, s, NULL, true);
> +                             pl011_event, pl011_be_change, s, NULL, true);
>   }
>   
>   static void pl011_reset(DeviceState *dev)