Re: [Qemu-devel] [PATCH 1/5] char: Let the caller know how many bytes were written in case of incomplete writes

[Paul Brook <paul@codesourcery.com>]

> On (Mon) Apr 05 2010 [17:33:38], Paul Brook wrote:
> > > There might be cases where a few bytes would have been sent out to char
> > > devices and some not. Currently the return values from qemu_chr_write()
> > > to char devs are only -1, indicating an error, or the complete length
> > > of the string passed.
> > >
> > > Make 'len' a pointer instead, and indicate how much of the string was
> > > written. The return value will either be the same as 'len' or a
> > > negative number indicating an error condition.
> >
> > This seems wrong. We should not be getting recoverable errors.
> 
> I was thinking of adding a bool to CharDriverState to indicate if EAGAIN
> should be reported to the writer. This can be conveyed at the time of
> doing qemu_chr_add_handlers().
> 
> It would certainly be beneficial for consumers of virtio-serial to be
> notified of -EAGAIN so that the guest can be throttled till the chardev
> catches up with the data being sent.

EAGAIN should only ever occur if no bytes are written. If a stall condition 
occurs after some data has been written (and we allow partial completion) then 
this is indicated by returning a short count.

Paul