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7cf7d9b83d
[ Upstream commit 46243b85b0ec5d2cee7545e5ce18c015ce91957e ]
The position reporting on Intel Skylake and later chips via
azx_get_pos_skl() contains a udelay(20) call for the capture streams.
A call for this alone doesn't sound too harmful. However, as the
pointer PCM ops is one of the hottest path in the PCM operations --
especially for the timer-scheduled operations like PulseAudio -- such
a delay hogs CPU usage significantly in the total performance.
The code there was taken from the original code in ASoC SST Skylake
driver blindly. The udelay() is a workaround for the case where the
reported position is behind the period boundary at the timing
triggered from interrupts; applications often expect that the full
data is available for the whole period when returned (and also that's
the definition of the ALSA PCM period).
OTOH, HD-audio (legacy) driver has already some workarounds for the
delayed position reporting due to its relatively large FIFO, such as
the BDL position adjustment and the delayed period-elapsed call in the
work. That said, the udelay() is almost superfluous for HD-audio
driver unlike SST, and we can drop the udelay().
Though, the current code doesn't guarantee the full period readiness
as mentioned in the above, but rather it checks the wallclock and
detects the unexpected jump. That's one missing piece, and the drop
of udelay() needs a bit more sanity checks for the delayed handling.
This patch implements those: the drop of udelay() call in
azx_get_pos_skl() and the more proper check of hwptr in
azx_position_ok(). The latter change is applied only for the case
where the stream is running in the normal mode without
no_period_wakeup flag. When no_period_wakeup is set, it essentially
ignores the period handling and rather concentrates only on the
current position; which implies that we don't need to care about the
period boundary at all.
Fixes:
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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.