On Mon, May 17, 2021 at 09:40:45AM +0100, Jonathan Cameron wrote: > On Fri, 14 May 2021 11:37:12 -0700 > Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > > > On Fri, May 14, 2021 at 1:50 AM Jonathan Cameron > > <Jonathan.Cameron@xxxxxxxxxx> wrote: > > [..] > > > > If it simplifies the kernel implementation to assume single > > > > kernel-initiator then I think that's more than enough reason to block > > > > out userspace, and/or provide userspace a method to get into the > > > > kernel's queue for service. > > > > > > This last suggestion makes sense to me. Let's provide a 'right' way > > > to access the DOE from user space. I like the idea if it being possible > > > to run CXL compliance tests from userspace whilst the driver is loaded. > > > > Ah, and I like your observation that once the kernel provides a > > "right" way to access DOE then userspace direct-access of DOE is > > indeed a "you get to keep the pieces" event like any other unwanted > > userspace config-write. > > > > > Bjorn, given this would be a generic PCI thing, any preference for what > > > this interface might look like? /dev/pcidoe[xxxxxx].i with ioctls similar > > > to those for the BAR based CXL mailboxes? > > > > (warning, anti-ioctl bias incoming...) > > I feel very similar about ioctls - my immediate thought was to shove this in > debugfs, but that feels the wrong choice if we are trying to persuade people > to use it instead of writing code that directly accesses the config space. > > > > > Hmm, DOE has an enumeration capability, could the DOE driver use a > > scheme to have a sysfs bin_attr per discovered object type? This would > > make it simliar to the pci-vpd sysfs interface. > > We can discover the protocols, but anything beyond that is protocol > specific. I don't think there is a enough info available by any standards > defined method. Also part of the reason to allow a safe userspace interface > would be to provide a generic interface for vendor protocols and things like > CXL compliance tests where we will almost certainly never provide a more > specific kernel interface. > > Whilst sysfs would work for CDAT, some protocols are challenge response rather > than simple read back and that really doesn't fit well for sysfs model. > If we get other protocols that are simple data read back, then I would > advocate giving them a simple sysfs interface much like proposed for CDAT > as it will always be simpler to use + self describing. > > On a lesser note it might be helpful to provide sysfs attrs for > what protocols are supported. The alternative is to let userspace run > the discovery protocol. Perhaps we can do this as a later phase. > > > > > Then the kernel could cache objects like CDAT that don't change > > outside of some invalidation event. > > It's been a while since I last saw any conversation on sysfs bin_attrs > but mostly I thought the feeling was pretty strongly against them for anything > but a few niche usecases. > > Feels to me like it would break most of the usual rules in a way vpd does > not (IIRC VPD is supposed to be a simple in the sense that if you write a value > to a writable part, you will read back the same value). > > +CC Greg who is a fount of knowledge in this area (and regularly + correctly > screams at the ways I try to abuse sysfs :) Note I don't think Dan was > suggesting implementing response / request directly, but I think that is > all we could do given DOE protocols can be vendor specific and the standard > discovery protocol doesn't let us know the fine grained support (what commands > within a given protocol). sysfs binary files are ONLY for pass-through things that go to/from userspace/hardware without the kernel touching them at all. Like raw PCI config descriptors. challenge/response type stuff, really does still fit the ioctl model, so that is a viable solution if needed. thanks, greg k-h
