Hi Alan, > On Jan 15, 2015, at 22:45 , One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx> wrote: > > On Thu, 15 Jan 2015 11:47:26 -0700 > Jason Gunthorpe <jgunthorpe@xxxxxxxxxxxxxxxxxxxx> wrote: >> It is a novel idea, my concern would be that embedding the FPGA in the >> DT makes it permanent unswappable kernel memory. >> Not having the kernel hold the FPGA is best for many uses. > > If you have a filesysytem before the FPGA is set up then it belongs in > the file system. As you presumably loaded the kernel from somewhere there > ought to be a file system (even an initrd). > Request firmware does not imply keeping it around. You can always re-request when reloading (although there’s a nasty big of caching that needs to be resolved with the firmware loader). >> Having the kernel hold the FPGA as a swppable file handle/mmap of some >> sort is next best (obviously the fs must be operating during resume) > > For a lot of hardware that gets somewhat hairy because you don't know > what the dependancies between devices are on the resume but yes. > >> Unswappable kernel memory is the worst choice > > There is another case here - which is where the firmware is somewhere > else physically. For example in PCI ROM space, or flash, but designed to > be loaded by the OS. > > One of the ideas rolling about is to put the device tree overlay blob in an EEPROM and then load it from there (not from the filesystem). >> I think to justify the ioctls you need a reason to have the context >> of a FD. sysfs is stateless, so if my process dies the kernel doesn't >> know. > > That isn't to say the stateless information doesn't belong in sysfs. Eg > you don't neccessarily want to open the device node and go through ioctls > just for bits of information that are interesting to reporting tools. > > (Imagine an 'lsfpga' tool for the kind of things you'd leave in sysfs) > >> Identifying the ioctls needed would probably clarify things. My >> rough start would be >> >> - Get status: programed, not programmed, error? >> Bitfile type? (eg Xilinx has nearly every permutation of bit/byte >> ordering) > > That's probably some kind of "what are you" ioctl that returns the > vendor/type information. > Can we please not use ioctls if possible. Configfs seems to work just fine for configuration and for any other higher speed API we should use read/write/mmap. Ioctls are a pain for scripting and interpreted languages usually. >> - Hand over to a DT overlay (how does this work?) Lock transfers >> from FD to kernel > > That bit isn't stateful so I would actually have expected something in > the kernel ABI along the lines of > > request_fpga(blah) > > which does > > if in use by user > EBUSY > lock it (all user opens will fail) > > and > > release_fpga(blah) > > and a sysfs node indicating its busy (and perhaps also what for if the > request_fpga passes a textual name) > >> Not sure about partial reconfiguration - clearly the kernel needs to >> know and check that the bitfiles are of the correct family, I wonder >> if the approach should be to program a basis on the FPGA which then >> creates a new FPGA device in the system that can accept the partial >> reconfiguration - this way the locking makes sense, the basis is >> locked by the kernel and devices and the overlay remains >> lockable/swappable/whatever by a dedicated /dev/ node ?? > > I think so. > > If you closed the device you have no idea what happened between the close > and a re-open, therefore you have no idea what the FPGA state is. > >> Just thinking aloud, I've never had a use case for partial >> reconfiguration. > > The API handles it but whether it needs to be there from day one I don't > know. > Making the API handle partial reconfiguration from day one might be pushing tricky. I don’t remember any case where I came across a need for it. We could do with a virtual FPGA topology; have a root FPGA that’s keeping the common unchanged bits and another that’s keeping the bits that do change. Dunno how nice it would be in practice though. > Alan > Regards — Pantelis -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
