[Qemu-devel] [Bug 1381846] [NEW] Data sent to parallel port in guest is lost if host buffer fills up

[Adam Goode]

Public bug reported:

It appears that qemu will blindly write characters out to the chardev
and drop them on the floor if a write fails with EAGAIN, without
initiating flow control (via BUSY and ACK) back to the guest. If the
host buffer is too small, or is talking to a hardware device that is too
slow, data will be lost.

I notice this problem when I run a DOS program with this on the qemu command 
line:
-parallel /dev/usb/lp0

I can work around this problem by buffering via a pipe, but this looks
like a general problem. Is there a way to wire up the readiness of the
output chardev to the parallel port ACK and BUSY lines, and signal an
ISA interrupt? I don't know the code well enough to tell.

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1381846

Title:
  Data sent to parallel port in guest is lost if host buffer fills up

Status in QEMU:
  New

Bug description:
  It appears that qemu will blindly write characters out to the chardev
  and drop them on the floor if a write fails with EAGAIN, without
  initiating flow control (via BUSY and ACK) back to the guest. If the
  host buffer is too small, or is talking to a hardware device that is
  too slow, data will be lost.

  I notice this problem when I run a DOS program with this on the qemu command 
line:
  -parallel /dev/usb/lp0

  I can work around this problem by buffering via a pipe, but this looks
  like a general problem. Is there a way to wire up the readiness of the
  output chardev to the parallel port ACK and BUSY lines, and signal an
  ISA interrupt? I don't know the code well enough to tell.

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