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From: Jiahuan Zhang <jiahuanzhang90@gmail.com>
Date: Tue, 21 Mar 2017 19:33:52 +0100
Message-ID: <CAJy91CbNq+1XNgGyvPv+2WHqHbOu3Ee7xcaLSKF0TKB5poAPSg@mail.gmail.com>
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Subject: Re: [Qemu-devel] Guest application reading from pl011 without
 device driver
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: =?UTF-8?B?TWFyYy1BbmRyw6kgTHVyZWF1?= <marcandre.lureau@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>

Okay. Thank you very much.
I'll take your advice and test.
Let's see the chardev maintainers reply.

Best,
Huan

2017=E5=B9=B43=E6=9C=8821=E6=97=A5 19:28=EF=BC=8C"Peter Maydell" <peter.may=
dell@linaro.org>=E5=86=99=E9=81=93=EF=BC=9A

> On 21 March 2017 at 18:11, Jiahuan Zhang <jiahuanzhang90@gmail.com> wrote=
:
> > What do you mean "RXFE will be cleared"?
> > Here RXFE is
> > #define PL011_RXFE   0x10
>
> I mean that as soon as 1 byte of data is ready to read the
> RXFE bit will be cleared in the FR register. So you need:
>
> /* probably you want to actually detect EOF somehow but let's
>  * just read 16 bytes
>  */
> for (i =3D 0; i < 16; i++) {
>     while (*UART_FR & PL011_RXFE) {
>         /* loop until data available */
>     }
>     /* now read 1 byte, that's all we can be certain is there */
>     data[i] =3D *UART_DR;
>     /* ...and then go back and check RXFE again */
> }
>
> >> Are you saying that QEMU is looping round indefinitely calling
> >> pl011_can_receive() and never running the guest at all?
> >
> >
> > Yes, exactly when fifo is full, s->count =3D 16. pl011_can_recieve keep=
s
> > returning 0.
> > The guest program is blocked.
>
> Right, but if can_receive returns 0, then QEMU should decide
> "OK, can't do anything here" and run the guest.
>
> > I am using a windows named pipe to get the data from a window
> > host program, which uses ReadFile () in char_win.c
>
> OK, bugs in the windows-specific char backend would be
> unsurprising.
>
> I'm not entirely sure how the chardev layer works, but
> at the pl011 end if we return 0 from our can_receive
> function then the chardev layer should decide it has
> nothing to do until the pl011 later calls
> qemu_chr_fe_accept_input(), I think.
>
> I've cc'd Paolo and Marc-Andr=C3=A9 Lureau as the chardev
> maintainers.
>
> thanks
> -- PMM
>