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Re: [Qemu-arm] [PATCH] pl011: do not put into fifo before enabled the in

From: Wei Xu
Subject: Re: [Qemu-arm] [PATCH] pl011: do not put into fifo before enabled the interruption
Date: Fri, 26 Jan 2018 17:05:14 +0000
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

Hi Peter,

On 2018/1/26 16:36, Peter Maydell wrote:
> On 26 January 2018 at 16:00, Wei Xu <address@hidden> wrote:
>> If the user pressed some keys in the console during the guest booting,
>> the console will be hanged after entering the shell.
>> Because in the above case the pl011_can_receive will return 0 that
>> the pl011_receive will not be called. That means no interruption will
>> be injected in to the kernel and the pl011 state could not be driven
>> further.
>> This patch fixed that issue by checking the interruption is enabled or
>> not before putting into the fifo.
>> Signed-off-by: Wei Xu <address@hidden>
>> ---
>>  hw/char/pl011.c | 2 ++
>>  1 file changed, 2 insertions(+)
>> diff --git a/hw/char/pl011.c b/hw/char/pl011.c
>> index 2aa277fc4f..6296de9527 100644
>> --- a/hw/char/pl011.c
>> +++ b/hw/char/pl011.c
>> @@ -229,6 +229,8 @@ static int pl011_can_receive(void *opaque)
>>      PL011State *s = (PL011State *)opaque;
>>      int r;
>> +    if (!s->int_enabled)
>> +       return 0;
>>      if (s->lcr & 0x10) {
>>          r = s->read_count < 16;
>>      } else {
>> --
> This doesn't look right. You should be able to use the PL011
> in a strictly polling mode, without ever enabling interrupts.
> Returning false in can_receive if interrupts are disabled
> would break that.
> If the user presses keys before interrupts are enabled,
> what ought to happen is:
>  * we put the key in the FIFO, and update the int_level flags
>  * when the FIFO is full, can_receive starts returning 0 and
>    QEMU stops passing us new characters
>  * when the guest driver for the pl011 initializes the
>    device and enables interrupts then either:
>     (a) it does something that clears the FIFO, which will
>     mean can_receive starts allowing new chars again, or
>     (b) it leaves the FIFO as it is, and we should thus
>     immediately raise an interrupt for the characters still
>     in the FIFO; when the guest handles this interrupt and
>     gets the characters, can_receive will permit new ones

Yes, now it is handled like b.

> What is happening in your situation that means this is not
> working as expected ?

But in the kernel side, the pll011 is triggered as a level interruption.
During the booting, if any key is pressed ,the call stack is as below:
QEMU side:
-->qemu_set_irq(level as 0)

Kernel side:
---->vgic_validate_injection (if level did not change, return)

Without above changes, in the vgic_validate_injection, because the
interruption level is always 0, this irq will not be queued into vgic.
And the guest will not read the pl011 fifo.

Best Regards,

> thanks
> -- PMM
> .

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