View source for IDE No Probe Specification


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== Introduction ==

This is the specification for a change in behavior of Linux support for the IDE "noprobe" option.

Historically, the IDE driver supports command line options to avoid probing certain devices and interfaces. 
  /\ ''Note - The command line options for the IDE driver are documented in the
  file <code>drivers/ide/ide.c</code>, in a comment right before the routine ide_setup()
  (at about line 3200 in the file).''

The "ide<x>=noprobe" option was found not to work as expected in the in the 2.4.20 code.

The basic problem is that the data structures that describe possible IDE interfaces are
initialized to indicate "noprobe" when the option is set at the kernel command line.
However, later some drivers reset the value of "noprobe" in the data structures
when certain interfaces are detected.

The end result is that (at least in Linux version 2.4.20) the kernel command line
processing for "ide<x>=noprobe" has no practical effect.

The desired behavior is that the "noprobe" option will be observed, even if 
subsequent driver initialization finds out that an interface is actually present.

== Rationale ==
Todd Poynor of [[Monta Vista]] measured the effect of specifying "hdf=none ide3=noprobe" (avoid probing ide2 slave and both ide3 devices) on a 200MHz IBM 405GP "Walnut" evaluation board with a 33MHz PCI bus. A Seagate Barracuda ATA IV 60GB disk drive was cabled to one of the two IDE interfaces on a Promise Ultra66 PCI-IDE bridge card (PDC20262 chipset). All of the drivers for PCI, IDE, PCI-IDE disk, and EXT2 file system were built into the kernel. 

The time to initialize IDE was 1.3 seconds when the missing devices were probed, and about 230 milliseconds when "hdf=none ide3=noprobe" was specified. Thus the resulting bootup time savings, with
"noprobe" properly observed by the Linux kernel, were about 1.1 seconds, in this test. 

This specification is needed because the kernel sometimes fails to observe the "noprobe"
command line option.

== Specifications ==
# When the "ide<x>=noprobe" option is specified on the kernel command line, the IDE driver in the Linux kernel SHALL avoid probing the specified IDE interface. 2. When the "hd<x>=noprobe" option is specified on the kernel command line, the IDE driver in the Linux kernel SHALL avoid probing the specified IDE device.

== Notes (informational and non-normative) ==
# Processing for the option "hd<x>=none" (which is related to the "hd<x>=noprobe" option) is specific to a single processor architecture (i386).  No aspect of the behavior of this option is specified here.

== References ==
See the page [http://tree.celinuxforum.org/pubwiki/moin.cgi/IDENoProbe IDENoProbe] for more
information about this problem, and for a sample implementation.

== Remaining Issues ==
 - see if "noprobe" behavior has changed in 2.6
[[Category:Specification]]