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The control elements are managed in a single linked list and we
traverse the whole list for matching each numid or ctl id per every
inquiry of a control element. This is OK-ish for a small number of
elements but obviously it doesn't scale. Especially the matching with
the ctl id takes time because it checks each field of the snd_ctl_id
element, e.g. the name string is matched with strcmp().
This patch adds the hash tables with Xarray for improving the lookup
speed of a control element. There are two xarray tables added to the
card; one for numid and another for ctl id. For the numid, we use the
numid as the index, while for the ctl id, we calculate a hash key.
The lookup is done via a single xa_load() execution. As long as the
given control element is found on the Xarray table, that's fine, we
can give back a quick lookup result. The problem is when no entry
hits on the table, and for this case, we have a slight optimization.
Namely, the driver checks whether we had a collision on Xarray table,
and do a fallback search (linear lookup of the full entries) only if a
hash key collision happened beforehand.
So, in theory, the inquiry for a non-existing element might take still
time even with this patch in a worst case, but this must be pretty
rare.
The feature is enabled via CONFIG_SND_CTL_FAST_LOOKUP, which is turned
on as default. For simplicity, the option can be turned off only when
CONFIG_EXPERT is set ("You are expert? Then you manage 1000 knobs").
Link: https://lore.kernel.org/r/20211028130027.18764-1-tiwai@suse.de
Link: https://lore.kernel.org/r/20220609180504.775-1-tiwai@suse.de
Link: https://lore.kernel.org/all/cover.1653813866.git.quic_rbankapu@quicinc.com/
Link: https://lore.kernel.org/r/20220610064537.18660-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.