Re: [PATCH v2] ASoC: soc-core: Create device_link to ensure correct suspend order

[Richard Fitzgerald <rf@opensource.cirrus.com>]

On 17/6/26 15:10, Marek Szyprowski wrote:
> Dear All,
> 
> On 11.06.2026 13:08, Richard Fitzgerald wrote:
>> In snd_soc_bind_card() create a device_link from card to all components
>> to ensure correct order of system_suspend. The card is the consumer and
>> the components are the supplier, so that the card will system_suspend
>> before any of the components.
>>
>> The PM core will normally system_suspend drivers in the opposite order
>> that they registered. This ensures children are suspended before their
>> parents, for example users of a bus driver should suspend before the bus
>> driver suspends.
>>
>> For ASoC, snd_soc_suspend() shuts down any active audio, which requires
>> that the components are still able to communicate with their hardware.
>> Previously there was nothing to ensure this ordering, because there is
>> (usually) no relationship between a machine driver and component drivers.
>> If the machine driver registered before the codec drivers, the codec
>> drivers would be suspended before the machine driver snd_soc_suspend()
>> runs, so that ASoC is attempting to stop audio on a driver that has
>> already suspended.
>>
>> Creating a device_link is safe if there is already a device_link between
>> those devices because of multiple components sharing the same dev.
>> device_link_add() kernel doc says:
>>
>>   "if a device link between the given @consumer and @supplier pair
>>    exists already when this function is called for them, the existing link
>>    will be returned regardless of its current type and status ...
>>    The caller of this function is then expected to treat
>>    the link as though it has just been created, so (in particular) if
>>    DL_FLAG_STATELESS was passed in @flags, the link needs to be released
>>    explicitly when not needed any more"
>>
>> For the same reason it is safe if the codec driver or machine driver
>> later call device_link_add() to create a link between the same two
>> devices.
>>
>> (I have tested creating multiple links between the card->dev and a
>> component->dev and did not encounter any problems with suspend/resume or
>> module unloading.)
>>
>> The DL_FLAG_AUTOREMOVE_* flags assume that they are being called from
>> the probe() function of that device. This isn't guaranteed in ASoC card
>> binding because of deferred binding. The exact behavior and consequences
>> of the DL_FLAG_AUTOREMOVE_* are also unclear from the documentation.
>> So DL_FLAG_STATELESS is used for safety, and the links are removed
>> explicitly when the card unbinds or if the bind fails.
>>
>> Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
>> ---
> 
> 
> This patch landed recently in linux-next as commit 0f54ce994b23 ("ASoC:
> soc-core: Create device_link to ensure correct suspend order"). In my
> tests I found that it breaks probing of VC4 DRM subsystem on Raspberry Pi
> 3 and 4 boards due to an issue with hdmi-audio-codec:
> 
> # dmesg | grep vc4
> vc4-drm gpu: bound fe400000.hvs (ops vc4_hvs_ops [vc4])
> vc4_hdmi fef00700.hdmi: Failed to create device link to hdmi-audio-codec.1.auto
> vc4_hdmi fef00700.hdmi: error -EINVAL: Could not register sound card
> vc4-drm gpu: failed to bind fef00700.hdmi (ops vc4_hdmi_ops [vc4]): -22
> vc4-drm gpu: adev bind failed: -22
> vc4-drm gpu: probe with driver vc4-drm failed with error -22
> 

Where in device_link_add() does it fail?