linux-kernel - Re: [RFC] of/platform: Create device link between simple-mfd and its children

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Message-ID: <0f0b7021e85a832afd42c6f9016158d6d8b0b28b.camel@suse.de>
Date:   Fri, 16 Oct 2020 17:26:56 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        Frank Rowand <frowand.list@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "moderated list:BROADCOM BCM2835 ARM ARCHITECTURE" 
        <linux-rpi-kernel@...ts.infradead.org>,
        Saravana Kannan <saravanak@...gle.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] of/platform: Create device link between simple-mfd and
 its children

On Fri, 2020-10-16 at 09:38 -0500, Rob Herring wrote:
> On Thu, Oct 15, 2020 at 6:43 AM Nicolas Saenz Julienne
> <nsaenzjulienne@...e.de> wrote:
> > 'simple-mfd' usage implies there might be some kind of resource sharing
> > between the parent device and its children.
> 
> It does? No! The reason behind simple-mfd was specifically because
> there was no parent driver or dependency on the parent. No doubt
> simple-mfd has been abused.

Fair enough, so we're doing things wrong. Just for the record, I'm looking at
RPi´s firmware interface:

	firmware: firmware {
		compatible = "raspberrypi,bcm2835-firmware", "simple-mfd";
		#address-cells = <1>;
		#size-cells = <1>;
		mboxes = <&mailbox>;

		firmware_clocks: clocks {
			compatible = "raspberrypi,firmware-clocks";
			#clock-cells = <1>;
		};

		reset: reset {
			compatible = "raspberrypi,firmware-reset";
			#reset-cells = <1>;
		};
		[...]
	};

Note that "raspberrypi,bcm2835-firmware" has a driver, it's not just a
placeholder. Consumer drivers get a handle to RPi's firmware interface through
the supplier's API, rpi_firmware_get(). The handle to firmware becomes
meaningless if it is unbinded, which I want to protect myself against.

A simpler solution would be to manually create a device link between both
devices ("raspberrypi,bcm2835-firmware" and "raspberrypi,firmware-clocks" for
example) upon calling rpi_firmware_get(). But I wanted to try addressing the
problem in a generic way first.

Regards,
Nicolas


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