Re: [PATCH] soundwire: qcom: Fix off-by-one error in port exhaustion check

[Srinivas Kandagatla <srinivas.kandagatla@oss.qualcomm.com>]

On 10/12/26 11:17 AM, Christopher Obbard wrote:
> Qualcomm SoundWire master ports are indexed from 1, with bit 0 of
> port_mask reserved. Valid port numbers therefore range from 1 to
> maxport.
> 
> Commit 6ccec91c3535 ("soundwire: qcom: Fix port exhaustion check in
> stream_alloc_ports") changed the port exhaustion check from
> pn > maxport to pn >= maxport which rejects the last valid port.
> 
> On RB3gen2 (QCS6490), two WSA883x speaker amplifiers are connected
> to swr2, which has 8 ports. Each amplifier uses one port for playback
> and another for voltage and current sensing. The right amplifier's
> VISENSE port is mapped to master port 8, which is rejected by the new
> check.
> 
> Since all ports in a SoundWire stream are allocated together during
> hw_params, this causes the stream to fail with:
> 
>     qcom-soundwire 3250000.soundwire: All ports busy
> 
> The codec DMA backend then returns -EBUSY, preventing speaker playback.
> Disabling VISENSE works around the issue but also disables speaker
> protection.
This comment is incorrect, we do not have speaker protection enabled on
any of the devices.
> 
> The original fix addressed an exhaustion check that could never
> trigger because find_first_zero_bit() returns maxport when all
> searched bits are set. Include the reserved bit 0 in the search so
> exhaustion returns maxport + 1 and restore the pn > maxport check.
> 
> Fixes: 6ccec91c3535 ("soundwire: qcom: Fix port exhaustion check in stream_alloc_ports")
Cc Stable and Reported-by missing

> Signed-off-by: Christopher Obbard <chris.obbard@oss.qualcomm.com>
> ---
>  drivers/soundwire/qcom.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/soundwire/qcom.c b/drivers/soundwire/qcom.c
> index 6e0c5fb10efad..b4ce96792fa94 100644
> --- a/drivers/soundwire/qcom.c
> +++ b/drivers/soundwire/qcom.c
> @@ -1348,9 +1348,9 @@ static int qcom_swrm_stream_alloc_ports(struct qcom_swrm_ctrl *ctrl,
>  				if (m_port)
>  					pn = m_port;
>  				else
> -					pn = find_first_zero_bit(port_mask, maxport);
> +					pn = find_first_zero_bit(port_mask, maxport + 1);
thanks for adding this case, rb3gen2 should not hit this anyway.
>  
> -				if (pn >= maxport) {
> +				if (pn > maxport) {
>  					dev_err(ctrl->dev, "All ports busy\n");
>  					return -EBUSY;
>  				}
> 

Once those are fixed,

Reviewed-by: Srinivas Kandagatla <srinivas.kandagatla@oss.qualcomm.com>

--srini
> ---
> base-commit: 2e9eff8847a46530ab937696f340b9d5bea2e4ef
> change-id: 20261012-wip-obbardc-soundwire-qcom-fix-port-off-by-one-47c0a8af1c5e
> 
> Best regards,
> --  
> Christopher Obbard
>