[Qemu-devel] Re: [PATCH 0/8] (v2) chardev, virtio-console: flow control, error handling, fixes

[Gerd Hoffmann]

On 04/08/10 00:58, Paul Brook wrote:
> > Hello,
> >
> > This patchset introduces flow control to virtio-console and
> > chardev-based virtio serial ports. This series is based on the
> > previous series I sent on Mar 31st (00/17: v4: virtio-serial fixes,
> > new abi for port discovery)
> >
> > The qemu chardevs can now return -EAGAIN when a non-blocking remote
> > isn't ready to accept more data.
> >
> > Comments?
>
> This is a major change in semantics. Are you sure all users handle this
> correctly? My guess is that most of the devices don't.

I don't expect trouble here.  EAGAIN is returned only for file handles 
in non-blocking mode.  I doubt existing users use non-blocking I/O as 
this makes the current unix_write() code go busy-loop in case the 
outgoing pipe is full.

> EAGAIN isn't really a useful response unless you have some way of notifying
> the device that it can send more data.

This is a valid point though.

cheers,
  Gerd