Re: [Qemu-devel] Guest application reading from pl011 without device driver

[Jiahuan Zhang]

On 21 March 2017 at 17:50, Peter Maydell <address@hidden> wrote:

> On 21 March 2017 at 16:47, Jiahuan Zhang <address@hidden> wrote:
> > The actual situation is, pl011_can_receive is returning false, the fifo
> is
> > full,   pl011_read does't start.
> > At this moment, only pl011_can_recieve is keeping returning 0.
>
> OK, so the incoming data has filled up the FIFO, and the UART
> is now sitting waiting for the guest to read the data...
>

Yes. And I think the guest reading can start when the fifo is nonempty.
In the guest program, I first check the flag register, once the fifo is
nonempty,
then move on to read from the data register.

This works to read small data, smaller than 16 bytes.

>
> >>  * whether pl011_can_receive is returning true even with
> >>    16 bytes in the fifo (hard to see how, given the code)
> >>  * whether the fifo actually has fewer bytes in it because
> >>    the guest is reading them
> >
> >
> > No, because the guest application does't get any data.
>
> ...so if the guest isn't getting any data that suggests it's
> the guest code's bug (ie it is not actually reading out of
> the FIFO).
>

What I found is pl011 expects to read all the data into fifo,
though pl011_can_receive returns false.
Only when all the data is read, pl011_read can start.

>
> thanks
> -- PMM
>