Re: [PATCH v3] Input: ads7846 - don't use scratch for tx_buf when clearing register

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

Hi Kris,

On Thu, Apr 30, 2026 at 05:37:38PM +0000, Kris Bahnsen wrote:
> The workaround for XPT2046 clears the command register, giving the
> touchscreen controller a NOP. The change incorrectly re-uses the
> req->scratch variable which is used as rx_buf for xfer[5], so by
> the time xfer[6] occurs, the contents of req->scratch may not be
> 0. It was found that the touchscreen controller can end up in
> a completely unresponsive state due to it being given a command
> the driver does not expect.
> 
> Instead, rely on the spi_transfer behavior of tx_buf being NULL to
> transmit all 0 bits and use the scratch variable for the rx_buf for
> both the 1 byte command to and 2 byte response from the controller.
> 
> This change was tested on real TSC2046 and ADS7843 controllers,
> but not the XPT2046 the workaround was originally created for.
> Confirming that the original modification to clear the command
> register does not impact either real controller.
> 
> Fixes: 781a07da9bb94 ("Input: ads7846 - add dummy command register clearing cycle")
> Cc: stable@vger.kernel.org
> Co-developed-by: Mark Featherston <mark@embeddedTS.com>
> Signed-off-by: Mark Featherston <mark@embeddedTS.com>
> Signed-off-by: Kris Bahnsen <kris@embeddedTS.com>
> ---
> 
> V1 -> V2: Don't use rx_buf when clearing command reg
> V2 -> V3: Modify original 2 xfer command to eliminate dev_err()
>           output on xfer with len and NULL buffers
> 
>  drivers/input/touchscreen/ads7846.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/input/touchscreen/ads7846.c b/drivers/input/touchscreen/ads7846.c
> index 4b39f7212d35c..488bcc8393293 100644
> --- a/drivers/input/touchscreen/ads7846.c
> +++ b/drivers/input/touchscreen/ads7846.c
> @@ -403,8 +403,7 @@ static int ads7846_read12_ser(struct device *dev, unsigned command)
>  	spi_message_add_tail(&req->xfer[5], &req->msg);
>  
>  	/* clear the command register */
> -	req->scratch = 0;
> -	req->xfer[6].tx_buf = &req->scratch;
> +	req->xfer[6].rx_buf = &req->scratch;

Sashiko (I believe correctly) pointed out that by doing this "scratch"
is now write only and this may cause DMA from the device stomp on
message status and other unrelated data that shares the same cacheline
with scracth. While it was already a problem before now it is even more
likely.

Since scratch is now write-only I believe moving it below "sample"
forces it into separate cacheline and fixes this problem. Could you
please try making this change?

Thanks.

-- 
Dmitry