mirror of https://gitee.com/openkylin/linux.git
167b400217
drm_vblank_restore() exists because certain power saving states can clobber the hardware frame counter. The way it does this is by guesstimating how many frames were missed purely based on the difference between the last stored timestamp vs. a newly sampled timestamp. If we should call this function before a full frame has elapsed since we sampled the last timestamp we would end up with a possibly slightly different timestamp value for the same frame. Currently we will happily overwrite the already stored timestamp for the frame with the new value. This could cause userspace to observe two different timestamps for the same frame (and the timestamp could even go backwards depending on how much error we introduce when correcting the timestamp based on the scanout position). To avoid that let's not update the stored timestamp at all, and instead we just fix up the last recorded hw vblank counter value such that the already stored timestamp/seq number will match. Thus the next time a vblank irq happens it will calculate the correct diff between the current and stored hw vblank counter values. Sidenote: Another possible idea that came to mind would be to do this correction only if the power really was removed since the last time we sampled the hw frame counter. But to do that we would need a robust way to detect when it has occurred. Some possibilities could involve some kind of hardare power well transition counter, or potentially we could store a magic value in a scratch register that lives in the same power well. But I'm not sure either of those exist, so would need an actual investigation to find out. All of that is very hardware specific of course, so would have to be done in the driver code. Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com> Cc: Daniel Vetter <daniel.vetter@ffwll.ch> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20210218160305.16711-1-ville.syrjala@linux.intel.com Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> |
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| arch | ||
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| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
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| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
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| README | ||
README
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.