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[Bug 1381846] Re: Data sent to parallel port in guest is lost if host bu
From: |
Launchpad Bug Tracker |
Subject: |
[Bug 1381846] Re: Data sent to parallel port in guest is lost if host buffer fills up |
Date: |
Tue, 19 Jan 2021 04:17:50 -0000 |
[Expired for QEMU because there has been no activity for 60 days.]
** Changed in: qemu
Status: Incomplete => Expired
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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:
Expired
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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