Re: [PATCH V4 16/16] ARM64: tegra: select PM_GENERIC_DOMAINS

[Kevin Hilman <khilman@baylibre.com>]

Thierry Reding <thierry.reding@gmail.com> writes:

> On Thu, Jan 14, 2016 at 12:11:39PM +0100, Arnd Bergmann wrote:
>> On Thursday 14 January 2016 11:29:24 Thierry Reding wrote:
>> > 
>> > It just occurred to me that none of these options really make much of a
>> > difference. As Jon mentioned once we merge this series a lot of features
>> > on Tegra will start to rely on PM_GENERIC_DOMAINS and hence PM. So if we
>> > do want to build a kernel with a maximum of Tegra features enabled (and
>> > I think a multi_v7_defconfig should include that) we'll end up with a PM
>> > dependency anyway, whether forced via select or implied via depends on.
>> > 
>> > I'm beginning to wonder if PM really should be an option these days. The
>> > disadvantages of making it optional do outweigh the advantages in my
>> > opinion. I'm not saying that, in general, it's totally useless to build
>> > a kernel that has no PM support, but for the more specific case where
>> > you would want to enable multi-platform support I don't think there's
>> > much practical advantage in allowing !PM. One of the most common build
>> > warnings are triggered because of this option. Also multi-platform
>> > kernels are really big already, so much so that I doubt it would make a
>> > significant difference if we unconditionally built PM support. Also the
>> > chances are that we'll be seeing more and more SoCs support PM and rely
>> > on it, much like Tegra would with the addition of this series.
>> > 
>> > I imagine that we could save ourselves a lot of headaches by simply
>> > enabling PM by default, whether that be via the PM Kconfig option or by
>> > selecting it from ARCH_TEGRA and any other architectures that may come
>> > to rely on it. Doing so would also reduce the amount of test coverage
>> > that we need to do, both at compile- and runtime.
>> 
>> I think this needs some investigation. As a general policy, we should
>> not grow the kernel image size when moving from a traditional ARM
>> platform to an ARCH_MULTIPLATFORM one.
>
> If we make ARCH_TEGRA select PM, then moving to a multi-platform kernel
> isn't automatically going to increase the image size. The image size is
> only going to increase if you select ARCH_TEGRA to be part of the multi
> platform image.
>
>> This is somewhat contradicted by how we already require CONFIG_OF
>> to be set for multiplatform kernels, and that adds around 80kb
>> to the image size.
>
> Yeah, there's also a fair amount of per-SoC code that can't be built as
> a module and which will be included in multi-platform images when the
> corresponding ARCH_* symbol is enabled. But I think that's inevitable
> given the purpose of multi-platform images.
>
>> Looking at just the defconfig files, these are the ones that currently
>> do not set CONFIG_PM:
>> 
>> build/acs5k_defconfig/.config:# CONFIG_PM is not set
>> build/acs5k_tiny_defconfig/.config:# CONFIG_PM is not set
>> build/axm55xx_defconfig/.config:# CONFIG_PM is not set
>> build/bcm2835_defconfig/.config:# CONFIG_PM is not set
>> build/clps711x_defconfig/.config:# CONFIG_PM is not set
>> build/ebsa110_defconfig/.config:# CONFIG_PM is not set
>> build/footbridge_defconfig/.config:# CONFIG_PM is not set
>> build/ks8695_defconfig/.config:# CONFIG_PM is not set
>> build/netwinder_defconfig/.config:# CONFIG_PM is not set
>> build/rpc_defconfig/.config:# CONFIG_PM is not set
>> build/u300_defconfig/.config:# CONFIG_PM is not set
>> build/vf610m4_defconfig/.config:# CONFIG_PM is not set
>> 
>> The only ones among these are are actually multiplatform are axm55xx,
>> bcm2835, and u300. I see no downsides of force-enabling PM for
>> any of those, so we could decide to 'select PM' from
>> CONFIG_ARCH_MULTIPLATFORM.
>
> ARCH_MULTIPLATFORM selecting PM would include PM unconditionally, even
> if none of the selected platforms require it. In my opinion an explicit
> select from platforms that require PM would be cleaner.

I agree.

Doing it this way also points you exactly at which arch(es) needs to be
disabled if you want to build a !PM multi-plaform kernel.

> It could be that once we start doing that for a single platform others
> might follow.

I suspect so as well.  The main reason we're not there already is that
full PM support for most platforms remains out of tree.

> When this becomes common place it might be worth moving it up a level,
> but I think explicit dependencies would be better for starters.

+1

Kevin