linux-kernel - Re: [PATCH 06/10] mfd: cs42l43: Add support for cs42l43 core driver

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Date:   Thu, 18 May 2023 17:15:57 +0100
From:   Richard Fitzgerald <rf@...nsource.cirrus.com>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        Charles Keepax <ckeepax@...nsource.cirrus.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
CC:     <broonie@...nel.org>, <lee@...nel.org>, <robh+dt@...nel.org>,
        <krzysztof.kozlowski+dt@...aro.org>, <conor+dt@...nel.org>,
        <tglx@...utronix.de>, <maz@...nel.org>, <linus.walleij@...aro.org>,
        <vkoul@...nel.org>, <lgirdwood@...il.com>,
        <yung-chuan.liao@...ux.intel.com>, <sanyog.r.kale@...el.com>,
        <alsa-devel@...a-project.org>, <patches@...nsource.cirrus.com>,
        <devicetree@...r.kernel.org>, <linux-gpio@...r.kernel.org>,
        <linux-spi@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/10] mfd: cs42l43: Add support for cs42l43 core driver



On 18/05/2023 16:16, Pierre-Louis Bossart wrote:
> 
>>>> +	ret = regmap_register_patch(cs42l43->regmap, cs42l43_reva_patch,
>>>> +				    ARRAY_SIZE(cs42l43_reva_patch));
>>>> +	if (ret) {
>>>> +		dev_err(cs42l43->dev, "Failed to apply register patch: %d\n", ret);
>>>> +		goto err;
>>>> +	}
>>>> +
>>>> +	pm_runtime_mark_last_busy(cs42l43->dev);
>>>> +	pm_runtime_put_autosuspend(cs42l43->dev);
>>>> +
>>>> +	ret = devm_mfd_add_devices(cs42l43->dev, PLATFORM_DEVID_NONE,
>>>> +				   cs42l43_devs, ARRAY_SIZE(cs42l43_devs),
>>>
>>> I don't why adding devices is not in probe. They use the same regmap
>>> right? So there will be no problem in probing them from MFD probe.
>>
>> Well except SoundWire is a bit of a special boy, the hardware is
>> not necessarily available in probe, the hardware is only available
>> at some point later when the device attaches. Doing it this way all
>> of the attaching (and various detach/attach cycles the device needs
>> during configuration) are over by the time the child drivers bind, so
>> they don't all need special code to handle that.
> 
> if the devices are added in the probe, then the regmap needs to be moved
> to cache-only and another special API would be needed to tell the MFD
> framework to turn the regmap cache-only off.
> 
> But if it's the same regmap, the regmap cache is handled in the
> SoundWire update_status callback so maybe  Krzysztof's proposal does work?

But you still can't access the hardware in probe(). So you'd have all
the child drivers probing but not able to talk to the hardware. The
child drivers would have to hook into the update_status() somehow so
they know when the peripheral has enumerated.
It's simpler to add them after the hardware has enumerated - they will 
be able to access the hardware in their probe().