On Thu, 3 Nov 2022 at 15:01, Thierry Reding <thierry.reding@xxxxxxxxx> wrote: > > On Thu, Nov 03, 2022 at 12:23:20PM +0000, Robin Murphy wrote: > > On 2022-11-03 04:38, Prathamesh Shete wrote: > > > In order to fully make use of the !IOMMU_API stub functions, make the > > > struct iommu_fwspec always available so that users of the stubs can keep > > > using the structure's internals without causing compile failures. > > > > I'm really in two minds about this... fwspecs are an internal detail of the > > IOMMU API that are meant to be private between individual drivers and > > firmware code, so anything poking at them arguably does and should depend on > > CONFIG_IOMMU_API. It looks like the stub for dev_iommu_fwspec_get() was only > > added for the sake of one driver that was misusing it where it really wanted > > device_iommu_mapped(), and has since been fixed, so if anything my > > preference would be to remove that stub :/ > > Tegra has been using this type of weak dependency on IOMMU_API mainly in > order to allow building without the IOMMU support on some old platforms > where people may actually care about the kernel size (Tegra20 systems > were sometimes severely constrained and don't have anything that we'd > call an IOMMU today). > > We have similar stubs in place for most other major subsystems in order > to allow code to simply compile out if the subsystem is disabled, which > is quite convenient for sharing code between platforms that may want a > given feature and other platforms that may not want it, without causing > too much of a hassle with compile-testing. I agree with the above. Moreover, the stubs make the code more portable/scalable and so it becomes easier to maintain. > > > I don't technically have much objection to this patch in isolation, but what > > I don't like is the direction of travel it implies. I see the anti-pattern > > is only spread across Tegra drivers, making Tegra-specific assumptions, so > > in my view the best answer would be to abstract that fwpsec dependency into > > a single Tegra-specific helper, which would better represent the nature of > > what's really going on here. > > I don't see how this is an anti-pattern. It might not be common for > drivers to need to reach into iommu_fwspec, so that might indeed be > specific to Tegra (for whatever reason our IP seems to want extra > flexibility), but the general pattern of using stubs is wide-spread, > so I don't see why IOMMU_API would need to be special. Again, I agree. Moreover, a "git grep CONFIG_IOMMU_API" indicates that the problem isn't specific to Tegra. The "#ifdef CONFIG_IOMMU_API" seems to be sprinkled across the kernel. I think it would be nice if we could improve the situation. So far, using stubs along with what the $subject patch proposes, seems to me to be the best approach. [...] Kind regards Uffe
