Re: [Qemu-trivial] [Qemu-devel] [PATCH 1/1] arm: virt: change GPIO trigger interrupt to pulse

[Wei Huang <wei@redhat.com>]

On 02/26/2016 09:06 AM, Peter Maydell wrote:
> On 26 February 2016 at 14:54, Shannon Zhao <shannon.zhao@linaro.org> wrote:
>> On 2016/2/26 20:53, Peter Maydell wrote:
>>> I don't understand why a 500ms pulse is better than a short one.
>>>
>> Oh, I just pick a value which seems like a real latency for pressing a
>> button. What's your suggestion?
> 
> I would prefer to avoid the pain of having a timer whose state
> needs to be migrated. It's unclear to me why a 500ms pulse
> will solve anything that an instantaneous pulse does not,
> so I'd like to better understand the problem first.

The problem we found with pulse was: only the last state change in GPIO
is received by guest VM. In other words, with 0(L)->1(H)->0(L) or
1(H)->0(L)->1(0), PL061 only sees the last state (0 and 1). I guess this
is because QEMU is non-preemptive. The solution is to have the following
steps:
  * qemu_set_irq(gpio_in, 1)
  * yeild to guest VM
  * qemu_set_irq(gpio_in, 0)

Is there any way to do so in QEMU without using timer?

> 
> thanks
> -- PMM
>