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DSB LUT register writes vs. palette anti-collision logic appear to interact in interesting ways: - posted DSB writes simply vanish into thin air while anti-collision is active - non-posted DSB writes actually get blocked by the anti-collision logic, but unfortunately this ends up hogging the bus for long enough that unrelated parallel CPU MMIO accesses start to disappear instead Even though we are updating the LUT during vblank we aren't immune to the anti-collision logic because it kicks in briefly for pipe prefill (initiated at frame start). The safe time window for performing the LUT update is thus between the undelayed vblank and frame start. Turns out that with low enough CDCLK frequency (DSB execution speed depends on CDCLK) we can exceed that. As we are currently using non-posted writes for the legacy LUT updates, in which case we can hit the far more severe failure mode. The problem is exacerbated by the fact that non-posted writes are much slower than posted writes (~4x it seems). To mititage the problem let's switch to using posted DSB writes for legacy LUT updates (which will involve using the double write approach to avoid other problems with DSB vs. legacy LUT writes). Despite writing each register twice this will in fact make the legacy LUT update faster when compared to the non-posted write approach, making the problem less likely to appear. The failure mode is also less severe. This isn't the 100% solution we need though. That will involve estimating how long the LUT update will take, and pushing frame start and/or delayed vblank forward to guarantee that the update will have finished by the time the pipe prefill starts... 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.