Thierry Reding <thierry.reding@xxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
