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max98090_interrupt() and max98090_pll_work() run in 2 different threads.
There are 2 possible races:
Note: M98090_REG_DEVICE_STATUS = 0x01.
Note: ULK == 0, PLL is locked; ULK == 1, PLL is unlocked.
max98090_interrupt max98090_pll_work
----------------------------------------------
schedule max98090_pll_work
restart max98090 codec
receive ULK INT
assert ULK == 0
schedule max98090_pll_work (1).
In the case (1), the PLL is locked but max98090_interrupt unnecessarily
schedules another max98090_pll_work.
max98090_interrupt max98090_pll_work max98090 codec
----------------------------------------------------------------------
ULK = 1
receive ULK INT
read 0x01
ULK = 0 (clear on read)
schedule max98090_pll_work
restart max98090 codec
ULK = 1
receive ULK INT
read 0x01
ULK = 0 (clear on read)
read 0x01
assert ULK == 0 (2).
In the case (2), both max98090_interrupt and max98090_pll_work read
the same clear-on-read register. max98090_pll_work would falsely
thought PLL is locked.
Note: the case (2) race is introduced by the previous commit ("ASoC:
max98090: exit workaround earlier if PLL is locked") to check the status
and exit the loop earlier in max98090_pll_work.
There are 2 possible solution options:
A. turn off ULK interrupt before scheduling max98090_pll_work; and turn
on again before exiting max98090_pll_work.
B. remove the second thread of execution.
Option A cannot fix the case (2) race because it still has 2 threads
access the same clear-on-read register simultaneously. Although we
could suppose the register is volatile and read the status via I2C could
be much slower than the hardware raises the bits.
Option B introduces a maximum 10~12 msec penalty delay in the interrupt
handler. However, it could only punish the jack detection by extra
10~12 msec.
Adopts option B which is the better solution overall.
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20191122073114.219945-4-tzungbi@google.com
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
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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.