linux-kernel - Re: [PATCH 1/1] soundwire: bus: Don't re-enumerate before status is UNATTACHED

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Date:   Tue, 30 Aug 2022 10:02:15 +0100
From:   Richard Fitzgerald <rf@...nsource.cirrus.com>
To:     <vkoul@...nel.org>, <yung-chuan.liao@...ux.intel.com>,
        <pierre-louis.bossart@...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 1/1] soundwire: bus: Don't re-enumerate before status is
 UNATTACHED

On 29/08/2022 10:44, 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>
> ---
>   drivers/soundwire/bus.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/soundwire/bus.c b/drivers/soundwire/bus.c
> index cb77da84a4f9..a3d3d66b3410 100644
> --- a/drivers/soundwire/bus.c
> +++ b/drivers/soundwire/bus.c
> @@ -766,6 +766,13 @@ static int sdw_program_device_num(struct sdw_bus *bus)
>   			if (sdw_compare_devid(slave, id) == 0) {
>   				found = true;
>   
> +				/*
> +				 * Don't re-enumerate a device until we've seen
> +				 * it UNATTACH.
> +				 */
> +				if (slave->status != SDW_SLAVE_UNATTACHED)
> +					break;

This should return. We're going to keep seeing this peripheral as the
next to be enumerated until we program its device ID.

> +
>   				/*
>   				 * Assign a new dev_num to this Slave and
>   				 * not mark it present. It will be marked