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It is possible that there are active wakelock references at the time we
are disabling the DMC wakelock mechanism. We need to deal with that in
two ways:
(A) Implement the missing step from Bspec:
The Bspec instructs us to clear any existing wakelock request bit
after disabling the mechanism. That gives a clue that it is okay to
disable while there are locks held and we do not need to wait for
them. However, since the spec is not explicit about it, we need
still to get confirmation with the hardware team. Let's thus
implement the spec and add a TODO note.
(B) Ensure a consistent driver state:
The enable/disable logic would be problematic if the following
sequence of events would happen:
1. Function A calls intel_dmc_wl_get();
2. Some function calls intel_dmc_wl_disable();
3. Some function calls intel_dmc_wl_enable();
4. Function A is done and calls intel_dmc_wl_put().
At (2), the refcount becomes zero and then (4) causes an invalid
decrement to the refcount. That would cause some issues:
- At the time between (3) and (4), function A would think that
the hardware lock is held but it could not be really held
until intel_dmc_wl_get() is called by something else.
- The call made to (4) could cause the refcount to become zero
and consequently the hardware lock to be released while there
could be innocent paths trusting they still have the lock.
To fix that, we need to keep the refcount correctly in sync with
intel_dmc_wl_{get,put}() calls and retake the hardware lock when
enabling the DMC wakelock with a non-zero refcount.
One missing piece left to be handled here is the following scenario:
1. Function A calls intel_dmc_wl_get();
2. Some function calls intel_dmc_wl_disable();
3. Some function calls intel_dmc_wl_enable();
4. Concurrently with (3), function A performs the MMIO in between
setting DMC_WAKELOCK_CFG_ENABLE and asserting the lock with
__intel_dmc_wl_take().
I'm mostly sure this would cause issues future display IPs if DMC
trap implementation was completely removed. We need to check with
the hardware team whether it would be safe to assert the hardware
lock before setting DMC_WAKELOCK_CFG_ENABLE to avoid this scenario.
If not, then we would have to deal with that via software
synchronization.
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241108130218.24125-12-gustavo.sousa@intel.com
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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 reStructuredText 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.