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During IGT testing it has been shown that the specification defined polling time of 1 us for FCLK_DONE, is sometimes not enough. The issue is still reproducible while disabling C-states through the PM QoS framework and also while disabling preemtion. From this the most plausible explanation is that the issue is due to a firmware flaw. As a workaround, it is better to wait a little bit longer for the FCLK_DONE to come around, than to leave with an DRM_ERROR and having FCLK_DONE at a randome time after. While spinning a list of igt tests prone to reproduce the issue the FCLK_DONE poll failed at approximately 2% of the invocations of the bdw_set_cdclk function. The longest poll time during this testing was measured to ~7us. So, the suggested new poll time of 100us is on the safe side. v2: Added more documentation about investigations done. Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102243 Signed-off-by: Marta Lofstedt <marta.lofstedt@intel.com> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> Signed-off-by: Imre Deak <imre.deak@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20170908132829.6312-1-marta.lofstedt@intel.com
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.