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Turns out the DSB indexed register write command has rather significant initial overhead compared to the normal MMIO write command. Based on some quick experiments on TGL you have to write the register at least ~5 times for the indexed write command to come out ahead. If you write the register less times than that the MMIO write is faster. So it seems my automagic indexed write logic was a bit misguided. Go back to the original approach only use indexed writes for the cases we know will benefit from it (indexed LUT register updates). Currently we shouldn't have any cases where this truly matters (just some rare double writes to the precision LUT index registers), but we will need to switch the legacy LUT updates to write each LUT register twice (to avoid some palette anti-collision logic troubles). This would be close to the worst case for using indexed writes (two writes per register, and 256 separate registers). Using the MMIO write command should shave off around 30% of the execution time compared to using the indexed write command. Cc: stable@vger.kernel.org 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 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.