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usb backend busy-loops due to runloop logic error #2664

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michaelrsweet opened this Issue Jan 9, 2008 · 2 comments

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michaelrsweet commented Jan 9, 2008

Version: 1.3.5
CUPS.org User: twaugh.redhat

A logic error in backend/runloop.c causes the usb backend to use very much more power than it needs to.

The situation that triggers this is that backendRunLoop() is entered with use_bc==0 and side_cb!=NULL.

When print_bytes==0 in this situation, I think that this piece of code:

 if (print_bytes || !use_bc)
   FD_SET(device_fd, &output);

is trying to add device_fd to the output set if the select() call will be skipped. This worked up until revision 6170, which added the side_cb parts.

A patch to fix the logic error is attached. Original bug report:
https://bugzilla.redhat.com/show_bug.cgi?id=426653

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michaelrsweet commented Jan 9, 2008

CUPS.org User: mike

Fixed in Subversion repository.

This actually affected all of the standard CUPS backends that used backendRunLoop() (parallel, serial, socket, usb), not just the USB backend.

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michaelrsweet commented Jan 9, 2008

"cups-runloop.patch":

diff -up cups-1.3.5/backend/runloop.c~ cups-1.3.5/backend/runloop.c
--- cups-1.3.5/backend/runloop.c~ 2007-08-22 19:34:34.000000000 +0100
+++ cups-1.3.5/backend/runloop.c 2008-01-09 15:37:44.000000000 +0000
@@ -216,7 +216,7 @@ backendRunLoop(
FD_SET(CUPS_SC_FD, &input);

 FD_ZERO(&output);
  • if (print_bytes || !use_bc)
  • if (print_bytes || (!use_bc && !side_cb))
    FD_SET(device_fd, &output);

if (use_bc || side_cb)

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