By Jonathan Corbet
March 3, 2009
As the 2.6.29 kernel development cycle draws toward its eventual close, it
is appropriate to look back at the internal API changes which have been
made. The following list cannot possibly be exhaustive, but, hopefully, it
captures the major points.
- The massive task credentials
patch set has been merged. This code reorganizes the handling of
process credentials (user ID, capabilities, etc.). One of the
immediate implications of this change is direct references to
credential-oriented fields in the task structure need to be changed;
for example, current->user->uid becomes
current_uid(). See Documentation/credentials.txt for a
description of the new API.
- The ftrace code has seen a lot of internal changes. The function
tracing feature has seen a number of improvements, and the developers
have added
mechanisms to profile the behavior of if statements,
provide function call graphs,
obtain user-space stack traces, and
follow CPU power-state transitions.
- Most of the callback functions/methods associated with the
net_device structure have been moved out of that structure
and into the new struct net_device_ops. In-tree drivers
have been converted to the new API.
- The priv field has been removed from struct
net_device; drivers should use netdev_priv() instead.
- The generic PHY layer now has power management support. To that end,
two new methods - suspend() and resume() - have been
added to struct phy_driver.
- The networking layer now supports large receive offload (or
"generic receive offload") operation.
- The NAPI API has been cleaned up somewhat; in particular, functions
like netif_rx_schedule(), netif_rx_schedule_prep(),
and netif_rx_complete() have lost the unneeded struct
net_device parameter.
- The poll() file operation is now allowed to sleep; see this article for more
information on this change.
- The CPU mask mechanism, used to represent sets of processors in the
system, is in the middle of being massively reworked. The problem is
that CPU masks were often put on the stack, but, as the number of
processors grows, the stack lacks room for the mask. The new API is designed to
get these masks off the stack, and to guard against anybody ever
trying to put one back. See this
posting by Rusty Russell for details on this work.
- An infrastructure for
asynchronous function calls has been merged. This code is still a
work in progress, though, and, for 2.6.29, it will not be activated in
the absence of the fastboot command-line parameter.
- The exclusive I/O memory
allocation functions have been merged.
- There is a new synchronous hash interface called "shash." It
simplifies the use of synchronous hash operations while allowing the
same tfm to be used simultaneously in different threads. All in-tree
users have been switched to the new API.
- The hrtimer code has been simplified with the removal of variable
modes for callback functions. All processing is now done in hardirq
context.
- A new set of LSM hooks has been added; these support pathname-based
security operations. With the merging of these hooks, one major
obstacle to the inclusion of security modules like AppArmor and TOMOYO
has been removed.
- The kernel will now refuse to build with GCC 4.1.0 or 4.1.1; those
versions have unfortunate bugs which prevent the building of a working
kernel. Versions 3.0 and 3.1 have also been deemed to be too old and
will not be supported in 2.6.29.
- Video4Linux drivers now use a separate v4l2_file_operations
structure to hold their VFS-like callbacks. The prototypes of a
number of these functions have been changed to remove the
inode argument.
- Video4Linux2 has also acquired a new "subdevice" concept, meant to
reflect the fact that video "devices" tend to be, in reality, a set of
cooperating devices. See the new
document for a description of how this mechanism works.
- Two new functions - stop_machine_create() and
stop_machine_destroy() - allow the independent creation of
the threads used by stop_machine(). That, in turn, lets
those threads be created before trying to actually stop the machine,
making that operation more resistant to failure.
- The exports for a number of SUNRPC functions have been changed to
GPL-only.
- The internal MTD (memory technology device) API has seen significant
changes aimed at supporting larger devices (those requiring 64-bit
sizes).
Developers interested in the history of kernel API changes can look at the LWN 2.6 API changes page. After a
period of unfortunate neglect, this page has been made current once again;
your editor promises to be a bit more diligent about maintaining this page
in the future.
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