linux-kernel - Re: Grafting old platform drivers onto a new DT kernel

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Message-ID: <yw1xh9kv5c60.fsf@unicorn.mansr.com>
Date:	Mon, 09 Nov 2015 17:13:11 +0000
From:	Måns Rullgård <mans@...sr.com>
To:	Mason <slash.tmp@...e.fr>
Cc:	Javier Martinez Canillas <javier@...hile0.org>,
	Andrew Lunn <andrew@...n.ch>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux ARM <linux-arm-kernel@...ts.infradead.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Jason Cooper <jason@...edaemon.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: Grafting old platform drivers onto a new DT kernel

Mason <slash.tmp@...e.fr> writes:

> On 09/11/2015 17:12, Måns Rullgård wrote:
>
>> Mason writes:
>> 
>>> On 09/11/2015 16:40, Måns Rullgård wrote:
>>>
>>>> The simplest solution for you is probably to add a quick and dirty DT
>>>> binding to the old driver.  If it doesn't use any driver-specific
>>>> platform data struct, you only need to set .of_match_table in the
>>>> struct platform_driver.  If there is a platform data struct, you'll also
>>>> need to write some code to populate it from DT properties.  It shouldn't
>>>> take more than a few minutes per driver in most cases.
>>>
>>> I'll try that approach, although I fear that "a few minutes per driver"
>>> is an optimistic assessment.
>> 
>> If the driver only needs an MMIO region and an IRQ, it is literally five
>> lines of code.
>
> It took me 7 days to figure out there were 2 lines missing in the
> interrupt controller driver.
>
> My problem is that I don't understand the platform API, nor the
> interaction with the DT API.
>
> Let me see...
>
> In arch/arm/mach-tangox/platform_dev.c
>
> static struct platform_device tangox_sdhci0_device = { ... };
> static struct platform_device tangox_sdhci1_device = { ... };
>
> static void tangox_init_sdhci(void)
> {
> 	if (tangox_sdio_enabled(0))
> 		platform_device_register(&tangox_sdhci0_device);
>
> 	if (tangox_sdio_enabled(1))
> 		platform_device_register(&tangox_sdhci1_device);
> }
>
> called from tangox_init_devices() which is marked arch_initcall.

Delete all of that.  The generic DT code will create the platform
devices based on the device tree.

> In the driver

Add something like this:

static const struct of_device_id tangox_sdio_dt_ids[] = {
	{ .compatible = "sigma,tangox-sdio" },
	{ }
};

> static struct platform_driver tangox_platform_sdio0 = {

	.probe		= sdhci_tangox_probe,

> 	.remove		= sdhci_tangox_remove,
> 	.suspend	= sdhci_tangox_suspend,
> 	.resume		= sdhci_tangox_resume,
> 	.driver		= {
> 		.name	= "tangox-sdhci",
> 		.owner	= THIS_MODULE,

		.of_match_table = tangox_sdio_dt_ids,

> 	},
> };

And that should be it.

> static int __init tangox_sdhci_drv_init(void) {
> 	return platform_driver_probe(&tangox_platform_sdio0, sdhci_tangox_probe);
> }
>
> static void __exit tangox_sdhci_drv_exit(void) {
> 	platform_driver_unregister(&tangox_platform_sdio0);
> }
>
> module_init(tangox_sdhci_drv_init);
> module_exit(tangox_sdhci_drv_exit);

You can replace those functions and module_init()/module_exit() with
module_platform_driver() if you want.

> The old way:
>
> 1) call platform_device_register() with a "struct platform_device"
> 2) call platform_driver_probe with a "struct platform_driver"
>
> The new way(?)
>
> The mess in 2) is hidden behind module_platform_driver?

module_platform_driver() is just a macro that creates standard driver
register/unregister functions much like the ones above.

> The platform_device_register() is done by the DT core?

Correct.

> The struct platform_driver requires a probe function?

That's the usual way.

-- 
Måns Rullgård
mans@...sr.com
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