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Re: [Qemu-devel] [PATCH] serial: do not trigger THR interrupt after writ
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH] serial: do not trigger THR interrupt after writing to IER |
Date: |
Thu, 11 Dec 2014 15:16:46 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0 |
On 10/12/2014 17:34, Paolo Bonzini wrote:
> This is responsible for failure of migration from 2.2 to 2.1, because
> thr_ipending is always one in practice. Calling serial_update_irq is
> the right thing to do indeed, because writing to IER could cause an
> interrupt to appear. However, there is no reason to set thr_ipending
> again.
>
> This was already reported in 2010. See this quote from
> https://lists.gnu.org/archive/html/qemu-devel/2010-03/msg01914.html:
>
>> The commit in r1049 (serial interrupt fix (Hampa Hug)) prevents
>> booting Digital Research DOSPlus. Following patch partially reverts
>> that commit and makes DOSPlus booting in QEMU again.
>
> Bochs does not check LSR_THRE in IER, and the log message in r1049 doesn't
> explain why the change was made in the first place.
>
> This does not change the migration format, so 2.2.0 -> 2.1 will remain
> broken but we can fix 2.2.1 -> 2.1 without breaking 2.2.1 <-> 2.2.0.
>
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
> hw/char/serial.c | 5 +----
> 1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/hw/char/serial.c b/hw/char/serial.c
> index ebcacdc..cf8e4e3 100644
> --- a/hw/char/serial.c
> +++ b/hw/char/serial.c
> @@ -350,10 +350,7 @@ static void serial_ioport_write(void *opaque, hwaddr
> addr, uint64_t val,
> s->poll_msl = 0;
> }
> }
> - if (s->lsr & UART_LSR_THRE) {
> - s->thr_ipending = 1;
> - serial_update_irq(s);
> - }
> + serial_update_irq(s);
> }
> break;
> case 2:
>
Nope, this breaks the Windows UART driver.
Paolo