On 26/10/2020 21:51, Rob Herring wrote: > On Thu, Oct 22, 2020 at 10:23 AM Tom Rini <trini@xxxxxxxxxxxx> wrote: >> On Fri, Oct 23, 2020 at 01:58:04AM +1100, David Gibson wrote: >>> On Thu, Oct 22, 2020 at 08:32:54AM -0400, Tom Rini wrote: >>>> On Thu, Oct 22, 2020 at 03:00:13PM +1100, David Gibson wrote: >>>>> On Wed, Oct 21, 2020 at 06:49:14PM -0400, Tom Rini wrote: > > [...] > >>>>>> But what does all of this _mean_ ? I kinda think I have an answer now. >>>>>> One of the things that sticks out is 6dcb8ba408ec adds a lot and >>>>>> 11738cf01f15 reduces it just a little. >>>>> >>>>> Ah, that's a tricky one. If we don't handle unaligned accesses we >>>>> instead get intermittent bug reports where it just crashes. >>>> >>>> We really need to talk about that then. There was a problem of people >>>> turning off the sanity check for making sure the entire device tree was >>>> aligned and then having everything crash. >>> >>> Ok... I'm not really sure where you're going with that thought. >> >> In my reading of the mailing list history of how this issue came up, >> it was someone was booting a dragonboard or something, and they (or >> rather, the board maintainer set by default) the flag to use the device >> tree wherever it is in memory and NOT to relocate it to a properly >> aligned address. This in turn lead to the kernel getting an unaligned >> device tree and everything crashing. The "I know what I'm doing" flag >> was set, violated the documented requirements for device trees need to >> reside in memory and everything blew up. >> >> After that it was noticed that there could be some internal >> mis-alignment and if you tried those accesses on a CPU that doesn't >> support doing those reads easily there could be problems, but that's not >> a common at all case (as noted by it not having been seen in practice). > > Nor a problem on many environments to begin with. More below... > >>>>> I suppose we could add an ASSUME_ALIGNED_ACCESS flag, and it will just >>>>> break for either an unaligned dtb (unlikely) or if you attempt to load >>>>> an unaligned value from a property (more likely, but don't add the >>>>> flag if you're not sure you don't need it). >>>> >>>> So long as it's abstracted in such a way that we don't grow the size of >>>> everything again, yes, that is the right way forward I think. >>> >>> All the ASSUME flags should be resolved at compile time (at least with >>> normal optimization levels enabled in the compiler), so testing for >>> those shouldn't increase size at all. If they do, something is wrong. >> >> I'm saying that how ever this new ASSUME flag is done, it needs to be >> done in such a way the compiler really will be smart about it. So >> something like making a new function that does fdt64_ld() if we aren't >> ASSUME_ALIGNED_ACCESS and fdt64_to_cpu() if we are >> ASSUME_ALIGNED_ACCESS. > > Ah, unaligned accesses again... To summarize, both performance and > size suffer with not doing unaligned accesses. > > Why not a HAS_UNALIGNED_ACCESS flag instead (or the inverse) that will > do unaligned accesses? That would be more aligned with what the system > can support rather than sanity checking associated with ASSUME_*. > > To repeat from last time, everything ARMv6 and up can do unaligned > accesses if enabled. But that requires the MMU to be enabled, doesn't it? If I read the ARM ARM correctly, unaligned accesses always trap on device memory, regardless of SCTLR.A. And without the MMU enabled everything is device memory. We compile U-Boot with -mno-unaligned-access/-mstrict-align to cope with that, and that most likely affects libfdt as well? Also some 32-bit ARM platforms run U-Boot proper with the MMU disabled all the time, and I know of at least the sunxi-aarch64 SPL running with the MMU off as well. Cheers, Andre > people care about. Whether that's enabled or not is up to how SCTLR.A > is configured. Last I checked, u-boot clears this. Don't know about > SPL case though. > > Rob >
