On Fri, May 12, 2023 at 11:25:32AM +0300, Ilpo Järvinen wrote: > On Thu, 11 May 2023, Lukas Wunner wrote: > > On Thu, May 11, 2023 at 10:55:06AM -0500, Bjorn Helgaas wrote: > > > I didn't see the prior discussion with Lukas, so maybe this was > > > answered there, but is there any reason not to add locking to > > > pcie_capability_clear_and_set_word() and friends directly? > > > > > > It would be nice to avoid having to decide whether to use the locked > > > or unlocked versions. > > > > I think we definitely want to also offer lockless accessors which > > can be used in hotpaths such as interrupt handlers if the accessed > > registers don't need any locking (e.g. because there are no concurrent > > accesses). > > > > So the relatively lean approach chosen here which limits locking to > > Link Control and Link Control 2, but allows future expansion to other > > registers as well, seemed reasonable to me. > > I went through every single use of these functions in the mainline tree > excluding LNKCTL/LNKCTL2 ones which will be having the lock anyway: > > - pcie_capability_clear_and_set_* > - pcie_capability_set_* > - pcie_capability_clear_* We're also performing RMW through pcie_capability_read_word() + pcie_capability_write_word() combos, see drivers/pci/hotplug/pciehp_hpc.c for examples. > Do you still feel there's a need to differentiate this per capability > given all the information above? What I think is unnecessary and counterproductive is to add wholesale locking of any access to the PCI Express Capability Structure. It's fine to have a single spinlock, but I'd suggest only using it for registers which are actually accessed concurrently by multiple places in the kernel. > spinlock + irq / work drivers/pci/pcie/pme.c: pcie_capability_set_word(dev, PCI_EXP_RTCTL, > spinlock + irq / work drivers/pci/pcie/pme.c: pcie_capability_clear_word(dev, PCI_EXP_RTCTL, [...] > What's more important though, isn't it possible that AER and PME RMW > PCI_EXP_RTCTL at the same time so it would need this RMW locking too > despite the pme internal spinlock? Yes that looks broken, so RTCTL would be another register besides LNKCTL and LNKCTL2 that needs protection, good catch. Thanks, Lukas
