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156abe2961
The pins on the Bay Trail SoC have separate input-buffer and output-buffer enable bits and a read of the level bit of the value register will always return the value from the input-buffer. The BIOS of a device may configure a pin in output-only mode, only enabling the output buffer, and write 1 to the level bit to drive the pin high. This 1 written to the level bit will be stored inside the data-latch of the output buffer. But a subsequent read of the value register will return 0 for the level bit because the input-buffer is disabled. This causes a read-modify-write as done by byt_gpio_set_direction() to write 0 to the level bit, driving the pin low! Before this commit byt_gpio_direction_output() relied on pinctrl_gpio_direction_output() to set the direction, followed by a call to byt_gpio_set() to apply the selected value. This causes the pin to go low between the pinctrl_gpio_direction_output() and byt_gpio_set() calls. Change byt_gpio_direction_output() to directly make the register modifications itself instead. Replacing the 2 subsequent writes to the value register with a single write. Note that the pinctrl code does not keep track internally of the direction, so not going through pinctrl_gpio_direction_output() is not an issue. This issue was noticed on a Trekstor SurfTab Twin 10.1. When the panel is already on at boot (no external monitor connected), then the i915 driver does a gpiod_get(..., GPIOD_OUT_HIGH) for the panel-enable GPIO. The temporarily going low of that GPIO was causing the panel to reset itself after which it would not show an image until it was turned off and back on again (until a full modeset was done on it). This commit fixes this. This commit also updates the byt_gpio_direction_input() to use direct register accesses instead of going through pinctrl_gpio_direction_input(), to keep it consistent with byt_gpio_direction_output(). Note for backporting, this commit depends on: commit |
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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 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.