Messages in this thread | | | Date | Tue, 13 May 2014 02:12:46 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCHv4 3/3] edac: altera: Add EDAC support for Altera SoC SDRAM Controller |
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On Mon, May 12, 2014 at 06:36:57PM -0500, tthayer@altera.com wrote: > + ptemp[0] = 0x5A5A5A5A; > + ptemp[1] = 0xA5A5A5A5; > + /* Clear the error injection bits */ > + regmap_write(drvdata->mc_vbase, CTLCFG, read_reg); > + /* Ensure it has been written out */ > + wmb(); > + > + /* > + * To trigger the error, we need to read the data back > + * (the data was written with errors above) > + * The ACCESS_ONCE macros are used to prevent the > + * compiler optimizing these reads out. > + */ > + reg = ACCESS_ONCE(ptemp[0]); > + read_reg = ACCESS_ONCE(ptemp[1]); > + /* Force Read */ > + rmb();
Right, I still am a bit unsure about this thing. So sure, we funnel ptemp through an asm() block which stops the optimizer but we assign the results to reg and read_reg, i.e. two local variables which aren't used in this function anymore.
Thus, I don't see what stops the compiler from discarding them along with the ACCESS_ONCE() reads at some point, when it becomes really smart. Basically killing
> + reg = ACCESS_ONCE(ptemp[0]); > + read_reg = ACCESS_ONCE(ptemp[1]);
as they have to effect.
Maybe we can do the reads in asm - this should be pretty safe.
I.e., something like this:
asm volatile("" : "=r" (reg), "=r" (read_reg), : "0" (ptemp[0]), "1" (ptemp[1]));
and it does it on x86 (did a small test program) by shuffling the values through registers so we definitely have the reads.
.loc 1 28 0 movl -48(%rbp), %edx # ptemp, D.2240 movl -44(%rbp), %eax # ptemp, D.2240 .loc 1 27 0 movl %edx, -28(%rbp) # reg, reg movl %eax, -32(%rbp) # read_reg, read_reg
Btw, if you only want to do the reads, you can simply tell gcc to put them in registers
asm volatile("" :: "r" (ptemp[0]), "r" (ptemp[1]));
and be done with it but I don't know how smart it is with register allocation so as to recognize that they're already in registers (when they already are in some registers) and discard the reads.
Fun stuff! :-)
-- Regards/Gruss, Boris.
Sent from a fat crate under my desk. Formatting is fine. --
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