Re: [PATCH v3 2/3] Input: omap-keypad: errata i689: Correct debouncetime

From: Felipe Balbi
Date: Mon Jul 29 2013 - 14:08:28 EST


On Mon, Jul 29, 2013 at 07:45:10PM +0300, Illia Smyrnov wrote:
> From: Axel Haslam <axelhaslam@xxxxxx>
> Set debounce time value to 12ms to workaround the errata.
> When a key is released for a time shorter than the debounce time,
> in-between 2 key press (KP1 and KP2), the keyboard state machine will go to
> idle mode and will never detect the key release (after KP1,and also after KP2),
> and thus will never generate a new IRQ indicating the key release.
> From the operating system standpoint, only a key press as been detected,
> and the key will keep on being printed on the screen until another or
> the same key is pressed again.When the failing state has been reached,
> the KBD_STATEMACHINE register will show "idle state" and the KBD_FULLCODE
> register won't be empty, this is the signature of the bug.There is no impact
> on the power consumption of the system as the state machine goes to IDLE state.
> First thing is to program the debounce time correctly:
> If X (us) is the maximum time of bounces when a key is pressed or released,
> and Y (us) is the minimum time of a key release that is expected to be detected,
> then the debounce time should be set to a value inbetween X and Y.
> In case it is still possible to get shorter than debounce time key-release
> events, then the only solution is to implement a software workaround:
> Before printing a second character on the screen, the software must check if
> the keyboard has hit the failing condition (cf signature of the bug above)
> or if the key is still really pressed and then take the appropriate actions.
> Silicon revisions impacted: OMAP4430 ES 1.0,2.0,2.1,2.2,2.3
> OMAP4460 ES 1.0,1.1
> OMAP5430 ES 1.0

if only these are affected, should you have revision check ? Also we
don't support OMAP5430 ES 1.0. I say this because, IMO, debouncing time
is likely to be board-specific and/or application-specific.

> diff --git a/drivers/input/keyboard/omap4-keypad.c b/drivers/input/keyboard/omap4-keypad.c
> index feab00f..e8bdc76 100644
> --- a/drivers/input/keyboard/omap4-keypad.c
> +++ b/drivers/input/keyboard/omap4-keypad.c
> @@ -62,8 +62,10 @@
> /* OMAP4 values */
> #define OMAP4_VAL_IRQDISABLE 0x0
> -#define OMAP4_VAL_PVT 0x7
> +
> +/* Errata i689: set debounce time value to 12ms */
> +#define OMAP4_VAL_PVT 0x6

to make this easier to understand, you might want to convert this into a
better macro that takes an argument in microseconds and you calculate
the debouncing time which should be written to HW.


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