linux-kernel - Re: [PATCH v2 3/5] soundwire: bus: Don't re-enumerate before status is UNATTACHED

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Date:   Mon, 12 Sep 2022 13:00:18 +0200
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Richard Fitzgerald <rf@...nsource.cirrus.com>, vkoul@...nel.org,
        yung-chuan.liao@...ux.intel.com, sanyog.r.kale@...el.com
Cc:     alsa-devel@...a-project.org, linux-kernel@...r.kernel.org,
        patches@...nsource.cirrus.com
Subject: Re: [PATCH v2 3/5] soundwire: bus: Don't re-enumerate before status
 is UNATTACHED



On 9/7/22 10:52, Richard Fitzgerald wrote:
> Don't re-enumerate a peripheral on #0 until we have seen and
> handled an UNATTACHED notification for that peripheral.
> 
> Without this, it is possible for the UNATTACHED status to be missed
> and so the slave->status remains at ATTACHED. If slave->status never
> changes to UNATTACHED the child driver will never be notified of the
> UNATTACH, and the code in sdw_handle_slave_status() will skip the
> second part of enumeration because the slave->status has not changed.
> 
> This scenario can happen because PINGs are handled in a workqueue
> function which is working from a snapshot of an old PING, and there
> is no guarantee when this function will run.
> 
> A peripheral could report attached in the PING being handled by
> sdw_handle_slave_status(), but has since reverted to device #0 and is
> then found in the loop in sdw_program_device_num(). Previously the
> code would not have updated slave->status to UNATTACHED because it had
> not yet handled a PING where that peripheral had UNATTACHED.
> 
> This situation happens fairly frequently with multiple peripherals on
> a bus that are intentionally reset (for example after downloading
> firmware).
> 
> Signed-off-by: Richard Fitzgerald <rf@...nsource.cirrus.com>

Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>

> ---
>  drivers/soundwire/bus.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/soundwire/bus.c b/drivers/soundwire/bus.c
> index 1cc858b4107d..6e569a875a9b 100644
> --- a/drivers/soundwire/bus.c
> +++ b/drivers/soundwire/bus.c
> @@ -773,6 +773,16 @@ static int sdw_program_device_num(struct sdw_bus *bus)
>  			if (sdw_compare_devid(slave, id) == 0) {
>  				found = true;
>  
> +				/*
> +				 * To prevent skipping state-machine stages don't
> +				 * program a device until we've seen it UNATTACH.
> +				 * Must return here because no other device on #0
> +				 * can be detected until this one has been
> +				 * assigned a device ID.
> +				 */
> +				if (slave->status != SDW_SLAVE_UNATTACHED)
> +					return 0;
> +
>  				/*
>  				 * Assign a new dev_num to this Slave and
>  				 * not mark it present. It will be marked