linux-kernel - Re: [PATCH 6/6] soundwire: qcom: do not probe devices before bus/link init

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Date:   Wed, 3 May 2023 10:00:17 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        Vinod Koul <vkoul@...nel.org>,
        Bard Liao <yung-chuan.liao@...ux.intel.com>,
        Sanyog Kale <sanyog.r.kale@...el.com>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Cc:     Patrick Lai <quic_plai@...cinc.com>
Subject: Re: [PATCH 6/6] soundwire: qcom: do not probe devices before bus/link
 init

On 01/05/2023 15:43, Pierre-Louis Bossart wrote:
> 
> 
> On 5/1/23 07:24, Krzysztof Kozlowski wrote:
>> On 20/04/2023 23:37, Pierre-Louis Bossart wrote:
>>>
>>>
>>> On 4/20/23 05:16, Krzysztof Kozlowski wrote:
>>>> Soundwire devices are supposed to be kept in reset state (powered off)
>>>> till their probe() or component bind() callbacks.  However if they are
>>>> already powered on, then they might enumerate before the master
>>>> initializes bus in qcom_swrm_init() leading to occasional errors like:
>>>
>>> The problem statement is really hard to follow.
>>>
>>> The peripheral can only be enumerated AFTER
>>> a) the manager starts the bus clock and transmitting PING frames
>>> b) the peripheral detects the sync words for 16 frames in a row.
>>> c) the peripheral reports as Attached in the Device0 slot
>>>
>>> That sequence holds whether the manager does the enumeration manually or
>>> relies on hardware-assisted autoenumeration. This is what the spec requires.
>>>
>>> So why can't the bus clock start be controlled by the manager driver,
>>> and started once all required initializations are done?
>>>
>>> I mean, there's got to be some sort of parent-child hierarchy with
>>> manager first, peripheral(s) second, I don't get how these steps could
>>> be inverted or race.
>>>
>>>>   qcom-soundwire 6d30000.soundwire-controller: Qualcomm Soundwire controller v2.0.0 Registered
>>>>   wcd938x_codec audio-codec: bound sdw:0:0217:010d:00:4 (ops wcd938x_sdw_component_ops)
>>>>   wcd938x_codec audio-codec: bound sdw:0:0217:010d:00:3 (ops wcd938x_sdw_component_ops)
>>>>   qcom-soundwire 6ad0000.soundwire-controller: swrm_wait_for_wr_fifo_avail err write overflow
>>>>
>>>> The problem primarily lies in Qualcomm Soundwire controller probe() sequence:
>>>> 1. request_threaded_irq()
>>>> 2. sdw_bus_master_add() - which will cause probe() and component bind()
>>>>    of Soundwire devices, e.g. WCD938x codec drivers.  Device drivers
>>>>    might already start accessing their registers.
>>>
>>> not if the bus clock hasn't started...
>>>
>>>> 3. qcom_swrm_init() - which initializes the link/bus and enables
>>>>    interrupts.
>>>
>>> if you can move the clock start in 3) then problem solved. Why can't
>>> this be done?
>>
>> Responding to all your three responses:
>> The clock is enabled in this 3. qcom_swrm_init(), so the old code to my
>> knowledge is written exactly how you expect.
>>
>> However even with stopped clock, the device enumerates at
>> sdw_bus_master_add(), before anything is enabled.
> 
> Erm, that's not physically possible...
> 
> The peripheral can report as attached and be enumerated by the manager,
> i.e. assigned a non-zero "Device Number" after the peripheral
> synchronizes on 16 frames with valid static and dynamic syncwords. That
> can only happen if there is a clock toggling and PING frames transmitted
> on the data line.
> 
> There's something else at play here.

Yes, I think you are right and that "else" is my limited knowledge on
the entire setup.

You gave me awesome hint in email before that probe != enumeration !=
initialization, however the wcd938x sound codec drivers were assuming
some steps are equal.

wcd938x comes with three devices on two drivers:
1. RX Soundwire device (wcd938x-sdw.c driver)
2. TX Soundwire device, which is used as regmap (also wcd938x-sdw.c driver)
3. platform device (wcd938x.c driver) - glue and component master,
actually having most of the code using TX Soundwire device regmap.

The probe of each RX and TX Soundwire devices added components, but that
this did not mean devices were enumerated, as you said.

Considering what Mark said about using regcache (and sync it), I am now
replacing entire solution with proper regcache handling device
enumeration after component bind.

Best regards,
Krzysztof