Re: [PATCH V5 0/4] gpio: tegra: Cleanups and support for debounce

From: Stephen Warren
Date: Tue May 03 2016 - 11:59:23 EST

On 05/02/2016 01:06 PM, Laxman Dewangan wrote:

On Tuesday 03 May 2016 12:14 AM, Stephen Warren wrote:
On 05/02/2016 11:58 AM, Laxman Dewangan wrote:

Toggling OE bit is something emulating the open drain here.

From the perspective of the external HW that's attached to the GPIO, I
believe there's no difference.

I think idea is that when we configure the pin in open drain then it
should be automatically handled by HW when we want to set pin state
high or low. When we set low, the pin should be driven and when high
then it should be tristated input. We should not need any direction bit

I don't imagine anything in the kernel cares, so long as the correct
logic level is present on the pin based on whatever GPIO API was last

I'd be very surprised if there wasn't hardware that could only
implement open-drain by this "emulation" method, so I'd be very
surprised if something prohibited that implementation style.

The emulation method implemented just to not drive high for open drain.
Recently, proper callback added for hw control for open drain and hence
emulation method is not needed for such HW.

I think if HW support the callback to implement the open drain then use
the HW method otherwise fallback to emulation method.

I don't see any benefit to that. It makes the code more complex without enabling any more features.

For reference, on Tegra124 and earlier, very few pins have open-drain control in HW (pinmux) whereas you can emulate it in the GPIO module for any pin. In Tegra210 and Tegra186, many pins have open-drain control in HW (pinmux) yet a good number still don't, yet you can still emulate this in the GPIO module for any pin.