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Re: [PATCH] hw/char/pl011: Output characters using best-effort mode

From: Peter Maydell
Subject: Re: [PATCH] hw/char/pl011: Output characters using best-effort mode
Date: Thu, 20 Feb 2020 10:10:25 +0000

On Thu, 20 Feb 2020 at 09:10, Marc Zyngier <address@hidden> wrote:
> On 2020-02-20 06:01, Gavin Shan wrote:
> > This fixes the issue by using newly added API
> > qemu_chr_fe_try_write_all(),
> > which provides another type of service (best-effort). It's different
> > from
> > qemu_chr_fe_write_all() as the data will be dropped if the backend has
> > been running into so-called broken state or 50 attempts of
> > transmissions.
> > The broken state is cleared if the data is transmitted at once.
> I don't think dropping the serial port output is an acceptable outcome.

Agreed. The correct fix for this is the one cryptically described
in the XXX comment this patch deletes:

-        /* XXX this blocks entire thread. Rewrite to use
-         * qemu_chr_fe_write and background I/O callbacks */

The idea is that essentially we end up emulating the real
hardware's transmit FIFO:
 * as data arrives from the guest we put it in the FIFO
 * we try to send the data with qemu_chr_fe_write(), which does
   not block
 * if qemu_chr_fe_write() tells us it did not send all the data,
   we use qemu_chr_fe_add_watch() to set up an I/O callback
   which will get called when the output chardev has drained
   enough that we can try again
 * we make sure all the guest visible registers and mechanisms
   for tracking tx fifo level (status bits, interrupts, etc) are
   correctly wired up

Then we don't lose data or block QEMU if the guest sends
faster than the chardev backend can handle, assuming the
guest is well-behaved -- just as with a real hardware slow
serial port, the guest will fill the tx fifo and then either poll
or wait for an interrupt telling it that the fifo has drained
before it tries to send more data.

There is an example of this in hw/char/cadence_uart.c
(and an example of how it works for a UART with no tx
fifo in hw/char-cmsdk-apb-uart.c, which is basically the
same except the 'fifo' is just one byte.)

You will also find an awful lot of XXX comments like the
above one in various UART models in hw/char, because
converting an old-style simple blocking UART implementation
to a non-blocking one is a bit fiddly and needs knowledge
of the specifics of the UART behaviour.

The other approach here would be that we could add
options to relevant chardev backends so the user
could say "if you couldn't connect to the tcp server I
specified, throw away data rather than waiting", where
we don't have suitable options already. If the user specifically
tells us they're ok to throw away the serial data, then it's
fine to throw away the serial data :-)

-- PMM

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