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

[Uwe Kleine-König]

On Fri, Oct 16, 2020 at 05:26:56PM +0200, Nicolas Saenz Julienne wrote:
> 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@xxxxxxx> 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.

IMHO rpi_firmware_get() should get a reference on the firmware device
(and call try_module_get()) which prevents unbinding it.

Best regards
Uwe


-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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