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The elan touchscreen datasheet says that the reset GPIO only needs to be asserted for 500us in order to reset the regulator. The problem is that some boards need a level shifter between the signals on the GPIO controller and the signals on the touchscreen. All of these extra components on the line can slow the transition of the signals. On one board, we measured the reset line and saw that it took almost 1.8ms to go low. Even after we bumped up the "drive strength" of the signal from the default 2mA to 8mA we still saw it take 421us for the signal to go low. In order to account for this let's lengthen the amount of time that we keep the reset asserted. Let's bump it up from 500us to 5000us. That's still a relatively short amount of time and is much safer. It should be noted that this fixes real problems. Case in point: 1. The touchscreen power rail may be shared with another device (like an eDP panel). That means that at probe time power might already be on. 2. In probe we grab the reset GPIO and assert it (make it low). 3. We turn on power (a noop since it was already on). 4. We wait 500us. 5. We deassert the reset GPIO. With the above case and only a 500us delay we saw only a partial reset asserted, which is bad. Giving it 5ms is overkill but feels safer in case someone else has a different level shifter setup. Note that bumping up the delay to 5000 means that some configs yell about using udelay(). We'll change to using usleep_range(). We give a small range here because: - This isn't a delay that happens very often so we don't need to worry about giving a big range to allow for power efficiency. - usleep_range() is known to almost always pick the upper bound and delay that long and we really don't want to slow down the power on of the touchscreen that much. Signed-off-by: Douglas Anderson <dianders@chromium.org> Reviewed-by: Matthias Kaehlcke <mka@chromium.org> Link: https://lore.kernel.org/r/20221208180603.v2.5.I6edfb3f459662c041563a54e5b7df727c27caaba@changeid Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com> |
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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.