On Mon, Oct 01, 2018 at 08:10:26PM +0200, Ard Biesheuvel wrote: > On 1 October 2018 at 19:56, Russell King - ARM Linux > <li...@armlinux.org.uk> wrote: > > On Sun, Sep 30, 2018 at 04:49:04AM +0200, Jason A. Donenfeld wrote: > >> Per the discussion about half-way down in [1], the kernel doesn't > >> actually support the ARMv3 ISA, but selects it for some ARMv4 ISA > >> hardware that benefits from ARMv3 code generation. > > > > The issue is to do with the half-word stores in the ARMv4 ISA, which > > need to be avoided on StrongARM RiscPC - the bus from the processor > > card (which was designed for ARM610 and ARM710) does not support > > anything except 8-bit and 32-bit accesses, so the 16-bit load/store > > instructions don't work correctly. > > > > Obviously, the reason for having the compiler use ARMv3 is to avoid > > those instructions which we have no other way to prevent - however, > > the use of ARMv3 with the assembler ensures that ldrh/strh are not > > accidentally used. > > > > We could argue that the ARMv3 assembly files are now stable, so the > > chances of ldrh/strh being introduced is low, which would make this > > change tolerable, but the commit message needs to spell out that > > we lose this protection. > > > >> Such a consideration, > >> then, only applies to the compiler but not to the assembler. This commit > >> passes -march=armv4 to the assembler in those cases, so that code > >> written for ARMv4 will continue to compile and run fine, without needing > >> module-specific asflags-y overrides. > > > > Note that "code written for ARMv4" will not be usable on this platform > > if it makes use of ldrh/strh, so depending on which instructions the > > assembler is complaining about, it could very well be a real "you're > > doing something wrong" case. > > > > The side effect of this patch is that such cases will now be hidden > > rather than evaluated on a case-by-case basis. > > > > Thanks for the insight. > > So Arnd's suggestion to switch to armv3-m would actually be feasible > then? The code in question does not use ldrh only umull, and so it > should build with armv3m as well. And we will even generate some > better code for RiscPC if we apply it to both the cflags and the > asflags.
Yep. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 14.8Mbps down 650kbps up According to speedtest.net: 13Mbps down 490kbps up