linux-kernel - Re: [PATCH 1/4] ASoC: soc-card: Add storage for PCI SSID

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Message-ID: <07967d0b-043b-2541-b293-5a61056339eb@opensource.cirrus.com>
Date:   Wed, 13 Sep 2023 14:29:13 +0100
From:   Richard Fitzgerald <rf@...nsource.cirrus.com>
To:     Mark Brown <broonie@...nel.org>,
        Amadeusz Sławiński 
        <amadeuszx.slawinski@...ux.intel.com>
CC:     <pierre-louis.bossart@...ux.intel.com>,
        <yung-chuan.liao@...ux.intel.com>, <kai.vehmanen@...ux.intel.com>,
        <peter.ujfalusi@...ux.intel.com>, <alsa-devel@...a-project.org>,
        <patches@...nsource.cirrus.com>, <linux-kernel@...r.kernel.org>,
        Cezary Rojewski <cezary.rojewski@...el.com>
Subject: Re: [PATCH 1/4] ASoC: soc-card: Add storage for PCI SSID

On 13/09/2023 13:58, Mark Brown wrote:
> On Wed, Sep 13, 2023 at 12:56:03PM +0200, Amadeusz Sławiński wrote:
>> On 9/12/2023 6:32 PM, Richard Fitzgerald wrote:
> 
>>> +#ifdef CONFIG_PCI
>>> +	/*
>>> +	 * PCI does not define 0 as invalid, so pci_subsystem_set indicates
>>> +	 * whether a value has been written to these fields.
>>> +	 */
>>> +	unsigned short pci_subsystem_vendor;
>>> +	unsigned short pci_subsystem_device;
>>> +	bool pci_subsystem_set;
>>> +#endif /* CONFIG_PCI */
>>> +
>>>    	char topology_shortname[32];
>>>    	struct device *dev;
> 
>> This looks bit weird to me, snd_soc_card is _generic_ struct which is not
>> device specific in any way, and now you add fields for PCI, can't this
>> somehow be done using drvdata or something?
> 
> You're right that it's a bit messy but if we use driver data then it
> becomes specific to a particular driver and there's a goal here to share
> with subfunction drivers.  I was thinking we could refactor to a union
> or otherwise extend if we find other users with a similar need.

Yes, I was trying to avoid multiple custom ways of passing the same
data around. A significant advantage of explicitly passing the SSID
(if it's available) rather than a more abstract identifier (such as a
char *) is that's it's very well defined exactly what a PCI SSID is so
we know we can use it verbatim. A more abstract identifier creates an
implied trust (or mistrust) between the machine driver and the component
receiving it whether it's unique and in a useful format.

I could de-ugly it a bit by moving it out into a separate struct/union
and having just a member of that struct type in snd_soc_card.

An alternative was to add a function like the existing
snd_soc_component_set_whatever() family but that means adding a callback
pointer to struct snd_soc_component_driver, which is creating more
space overhead than one value in the snd_soc_card.