On Thu, Dec 08, 2022 at 11:20:06AM -0800, Douglas Anderson wrote: > 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 we let's lengthen the amount of time that nit: s/we let's/we/ || s/we let's/let's/ no need to re-spin just for this > 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. > > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx> Reviewed-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
