linux-kernel - Re: [PATCH v2 3/5] ARM: bcm2835: add rpi power domain driver

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Date:	Mon, 07 Dec 2015 17:04:58 -0800
From:	Eric Anholt <eric@...olt.net>
To:	Kevin Hilman <khilman@...nel.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	linux-arm-kernel@...ts.infradead.org,
	linux-rpi-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Stephen Warren <swarren@...dotorg.org>,
	Lee Jones <lee@...nel.org>,
	Florian Fainelli <f.fainelli@...il.com>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alexander Aring <alex.aring@...il.com>,
	devicetree@...r.kernel.org, linux-pm@...r.kernel.org,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>
Subject: Re: [PATCH v2 3/5] ARM: bcm2835: add rpi power domain driver

Kevin Hilman <khilman@...nel.org> writes:

> Eric Anholt <eric@...olt.net> writes:
>
>> From: Alexander Aring <alex.aring@...il.com>
>>
>> This patch adds support for several power domains on Raspberry Pi,
>> including USB (so it can be enabled even if the bootloader didn't do
>> it), and graphics.
>>
>> This patch is the combined work of Eric Anholt (who wrote USB support
>> inside of the Raspberry Pi firmware driver, and wrote the non-USB
>> domain support) and Alexander Aring (who separated the original USB
>> work out from the firmware driver).
>>
>> Signed-off-by: Alexander Aring <alex.aring@...il.com>
>> Signed-off-by: Eric Anholt <eric@...olt.net>
>> ---
>>
>> v2: Add support for power domains other than USB, using the new
>>     firmware interface, reword commit message (changes by Eric)
>
> [...]
>
>> +/*
>> + * Firmware indices for the old power domains interface.  Only a few
>> + * of them were actually implemented.
>> + */
>> +#define RPI_OLD_POWER_DOMAIN_USB		3
>> +#define RPI_OLD_POWER_DOMAIN_V3D		10
>> +
>
> Is "old" the right word here?  Are there firmware versions that could be
> used instead?  What happens when the firwmware is updated next time?

Old is a good word.  It's the old interface.

As for what happens when the firmware is updated: Nothing.  The firmware
is updated all the time, and it maintains backwards compatibility.
Unless you mean "what happens when a newer, fancier power domain
interface is created and we need a name for the newer one" and the
answer is "this is a define entirely within the driver, and we can just
rename it when we want to."

> [...]
>
>> +	/*
>> +	 * Use the old firmware interface for USB power, so that we
>> +	 * can turn it on even if the firmware hasn't been updated.
>> +	 */
>> +	rpi_init_old_power_domain(rpi_domains, RPI_POWER_DOMAIN_USB,
>> +				  RPI_OLD_POWER_DOMAIN_USB, "USB");
>
> This seems a bit restrictive.
>
> To me, it seems that determining "old" or "new" (or revision of fw
> interface to use) should be described in DT, not hard-coded in the power
> domain driver.
>
> What about an additional DT property to describe that? or possibly
> another cell in the domain which could be used to optionally set
> old/legacy.

As the author and maintainer of the code, I don't feel it's restrictive.
The firmware protocol is defined and is guaranteed to continue to exist,
it's only useful for this platform, and defining a new set of custom
devicetree properties for it would only obfuscate the implementation.
DT is a useful tool for separating out the between-board differences for
the same piece of hardware across multiple implementations at different
addresses, while this is neither hardware nor in multiple
implementations at different addresses.

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