linux-kernel - Re: [PATCH] input: cyttsp5: Fix bitmask for touch buttons

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Message-Id: <74450737-fce2-4ba8-b764-ac22c21490ad@app.fastmail.com>
Date:   Wed, 01 Feb 2023 18:00:57 +1000
From:   Alistair <alistair@...stair23.me>
To:     "Richard Kjerstadius" <kjerstadius@...il.com>,
        linux-input@...r.kernel.org
Cc:     "Richard Kjerstadius" <richard.kjerstadius@...edyne.com>,
        "Linus Walleij" <linus.walleij@...aro.org>,
        "Dmitry Torokhov" <dmitry.torokhov@...il.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] input: cyttsp5: Fix bitmask for touch buttons

On Fri, 27 Jan 2023, at 8:29 PM, Richard Kjerstadius wrote:
> Prior to this patch, the bitmask ends up being 0x3, as opposed to 0x1
> which likely was the intention. The erroneous bit results in the driver
> reporting 2 different button activations in designs with 2 or more
> buttons.
> 
> To detect which button has been pressed, cyttsp5_btn_attention() uses a
> for loop to iterate through the input buffer, while shifting and
> applying a bitmask to determine the state for each button.
> Unfortunately, when the bitmask is 0x3 and there are multiple buttons,
> this procedure falls apart.
> 
> Consider a design with 3 buttons. Pressing the third button will result
> in a call to cyttsp5_btn_attention() with the input buffer containing
> 0x4 (binary 0100). In the first iteration of the for loop cur_btn_state
> will be:
> 
> (0x4 >> 0 * 1) & 0x3 = 0x4 & 0x3 = 0x0
> 
> This is correct. However, in the next iteration this happens:
> 
> (0x4 >> 1 * 1) & 0x3 = 0x2 & 0x3 = 0x2
> 
> Which means that a key event for key 1 is generated, even though it's
> not really active. In the third iteration, the loop detects the button
> that was actually pressed:
> 
> (0x4 >> 2 * 1) & 0x3 = 0x1 & 0x3 = 0x1
> 
> This key event is the only one that should have been detected, but it is
> accompanied by the preceding key. Ensuring the applied mask is 0x1
> solves this problem.
> 
> Signed-off-by: Richard Kjerstadius <richard.kjerstadius@...edyne.com>

Reviewed-by: Alistair Francis <alistair@...stair23.me>

Alistair

> ---
> drivers/input/touchscreen/cyttsp5.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/input/touchscreen/cyttsp5.c b/drivers/input/touchscreen/cyttsp5.c
> index 4a23d6231382..16caffa35dd9 100644
> --- a/drivers/input/touchscreen/cyttsp5.c
> +++ b/drivers/input/touchscreen/cyttsp5.c
> @@ -29,7 +29,7 @@
> #define CY_MAX_INPUT 512
> #define CYTTSP5_PREALLOCATED_CMD_BUFFER 32
> #define CY_BITS_PER_BTN 1
> -#define CY_NUM_BTN_EVENT_ID GENMASK(CY_BITS_PER_BTN, 0)
> +#define CY_NUM_BTN_EVENT_ID GENMASK(CY_BITS_PER_BTN - 1, 0)
>  
> #define MAX_AREA 255
> #define HID_OUTPUT_BL_SOP 0x1
> -- 
> 2.39.0
> 
>