Messages in this thread | | | From | Arnd Bergmann <> | Date | Tue, 30 Jul 2019 20:24:14 +0200 | Subject | Re: [PATCH] powerpc: workaround clang codegen bug in dcbz |
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On Tue, Jul 30, 2019 at 6:16 PM Segher Boessenkool <segher@kernel.crashing.org> wrote: > > On Tue, Jul 30, 2019 at 04:30:29PM +0200, Arnd Bergmann wrote: > > On Tue, Jul 30, 2019 at 3:49 PM Segher Boessenkool > > <segher@kernel.crashing.org> wrote: > > > > > > On Tue, Jul 30, 2019 at 09:34:28AM +0200, Arnd Bergmann wrote: > > > > Upon a second look, I think the issue is that the "Z" is an input argument > > > > when it should be an output. clang decides that it can make a copy of the > > > > input and pass that into the inline asm. This is not the most efficient > > > > way, but it seems entirely correct according to the constraints. > > > > > > Most dcb* (and all icb*) do not change the memory pointed to. The > > > memory is an input here, logically as well, and that is obvious. > > > > Ah, right. I had only thought of dcbz here, but you are right that using > > an output makes little sense for the others. > > > > readl() is another example where powerpc currently uses "Z" for an > > input, which illustrates this even better. > > in_le32 and friends? Yeah, huh. If LLVM copies that to the stack as > well, its (not byte reversing) read will be atomic just fine, so things > will still work correctly.
byteorder is fine, the problem I was thinking of is when moving the load/store instructions around the barriers that synchronize with DMA, or turning them into different-size accesses. Changing two consecutive 16-bit mmio reads into an unaligned 32-bit read will rarely have the intended effect ;-)
Arnd
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