On Monday 07 September 2015 15:36:27 Daniel Thompson wrote: > On 07/09/15 13:33, Arnd Bergmann wrote: > > On Monday 07 September 2015 11:30:11 Daniel Thompson wrote: > > > Generally speaking, the kernel should shield user space from ABI > > differences in the firmware and still provide the same user space ABI > > (possibly emulated, when the newer firmware removes features of the > > old one). Can you think of a case where a device firmware directly > > defines the user ABI without having kernel visible changes? > > I don't mean that the firmware ABI is exposed to userspace. > > I mean if you use the same filename for both ABIs (in /lib/firmware) > then kernel X with lose functionality if it is booted with a userspace > that has updated to the new firmware (maybe even crash if I can't detect > at runtime the version of the firmware is has loaded). That sort of > thing is the pain for distros which support booting with multiple > kernels (apt-get/dnf simply won't know when it is safe to update the > firmware binary). I see. This is the case I was interested in with tying the compatible string to the firmware blob: the compatible string tells us which device we are dealing with, and if that is implemented in firmware, then loading the firmware based on the same string will make everything work, while having a separate DT property can cause trouble as the driver now has to check two properties to find out what the ABI is. Another case is where a driver knows of two firmware ABIs for the same hardware and the boot loader does not care which one is used. In this case, we also don't want to encode the firmware name in DT, but the driver has to know anyway, and I can see two ways of doing this: a) have version information inside of the blob. This works if the driver developer also controls the image format (as is often the case). Another downside is that it fails if the firmware image uses an ABI that the (older) kernel does not understand. b) append the ABI version to the file name and try loading the firmware for the latest interface first, then go back to increasingly older file names that the driver still understands. This is already common practice in some drivers, regardless of matching the file name to DT. > >> Having lists of firmwares can certainly help solve this (providing the > >> list can be used to allow a driver to select for an ABI is supports). > >> However I afraid I find this example argues against having filenames in > >> DT at all because it seems odd to me that, for kernel and userspace to > >> adopt ABI B that must wait until the DT is updated to include support > >> for ABI B. The hardware didn't change... > > > > This is not what I was thinking of for a list of file names: I would not > > want a case where the kernel might pick between multiple incompatible > > files without knowing what they are, especially if the differences > > are ABI relevant. > > Nor me. > > Personally I prefer an "assertive" driver that imposes a filename for > the firmwares it needs. Thus if there is an ABI change the driver writer > can load a completely different firmware file without needing a DT > update. That said I'm not especially invested either way. > > However whatever solution is reached I would quite like to be able to > boot kernels from either side of an ABI break from a single userspace > (and get the new features when the kernel supports them). > > > > However, being very specific would let users intentionally install a > > firmware that is only used on a particular instance of a device that > > they want to behave differently, while the generic firmware would come > > preinstalled with the normal linux-firmware package. > > Not sure I follow this. You mean embedding filenames in DT makes it easy > to something like a vendor/reverse-engineering mode. The example I was thinking of is drivers/net/wireless/ath/ath10k/, which first attempts to load the firmware image based on the specific PCI vendor/device/subvendor/subdevice ID, then the vendor/device tuple and finally falls back to just the driver name (or something similar to that, I haven't looked up the exact method). Arnd -- To unsubscribe from this list: send the line "unsubscribe devicetree-spec" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
