linux-kernel - Re: [PATCH v5 7/7] Input: matrix_keypad

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Message-ID: <Z7YNKl4ljWFQEa-u@mt.com>
Date: Wed, 19 Feb 2025 17:56:10 +0100
From: Manuel Traut <manuel.traut@...com>
To: Markus Burri <markus.burri@...com>
Cc: linux-kernel@...r.kernel.org,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Marek Vasut <marek.vasut@...il.com>, linux-input@...r.kernel.org,
	devicetree@...r.kernel.org
Subject: Re: [PATCH v5 7/7] Input: matrix_keypad - detect change during scan

On Fri, Jan 10, 2025 at 06:49:06AM +0100, Markus Burri wrote:
> For a setup where the matrix keypad is connected over a slow interface
> (e.g. a gpio-expansion over i2c), the scan can take a longer time to read.
> 
> Interrupts need to be disabled during scan. And therefore changes in this
> period are not detected.
> To improve this situation, scan the matrix again if the row state changed
> during interrupts disabled.
> The rescan is repeated until no change is detected anymore.

This is a quirk for a bad hardware design. For 'good' hardware it adds
an additional read_row_state for no need. For even slower connected
GPIOs this will also not help much. However it is obvious that it will
be an improvement for some designs. 

Dmitry, would it make sense to make this configurable?

> Signed-off-by: Markus Burri <markus.burri@...com>
> 
> ---
>  drivers/input/keyboard/matrix_keypad.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/drivers/input/keyboard/matrix_keypad.c b/drivers/input/keyboard/matrix_keypad.c
> index fdb3499..e50a6fe 100644
> --- a/drivers/input/keyboard/matrix_keypad.c
> +++ b/drivers/input/keyboard/matrix_keypad.c
> @@ -125,6 +125,10 @@ static void matrix_keypad_scan(struct work_struct *work)
>  	const unsigned short *keycodes = input_dev->keycode;
>  	uint32_t new_state[MATRIX_MAX_COLS];
>  	int row, col, code;
> +	u32 init_row_state, new_row_state;
> +
> +	/* read initial row state to detect changes between scan */
> +	init_row_state = read_row_state(keypad);
>  
>  	/* de-activate all columns for scanning */
>  	activate_all_cols(keypad, false);
> @@ -173,6 +177,18 @@ static void matrix_keypad_scan(struct work_struct *work)
>  		keypad->scan_pending = false;
>  		enable_row_irqs(keypad);
>  	}
> +
> +	/* read new row state and detect if value has changed */
> +	new_row_state = read_row_state(keypad);
> +	if (init_row_state != new_row_state) {
> +		guard(spinlock_irq)(&keypad->lock);
> +		if (unlikely(keypad->scan_pending || keypad->stopped))
> +			return;
> +		disable_row_irqs(keypad);
> +		keypad->scan_pending = true;
> +		schedule_delayed_work(&keypad->work,
> +				      msecs_to_jiffies(keypad->debounce_ms));
> +	}
>  }
>  
>  static irqreturn_t matrix_keypad_interrupt(int irq, void *id)
> -- 
> 2.39.5
>