pj4 -marm breaks thumb ftrace
Russell King - ARM Linux
linux at arm.linux.org.uk
Thu Nov 12 02:38:24 PST 2015
On Thu, Nov 12, 2015 at 11:30:24AM +0100, Ard Biesheuvel wrote:
> Compiling pj4-cp0.c in thumb mode fails on the sub instruction in the
> following sequence:
> static void __init pj4_cp_access_write(u32 value)
> u32 temp;
> __asm__ __volatile__ (
> "mcr p15, 0, %1, c1, c0, 2\n\t"
> "mrc p15, 0, %0, c1, c0, 2\n\t"
> "mov %0, %0\n\t"
> "sub pc, pc, #4\n\t"
> : "=r" (temp) : "r" (value));
> I wonder if the sub instruction is simply there to flush the pipeline, but
> let's not get into that.
It is. The whole "mov %0, %0; sub pc, pc, #4" is a documented requirement
for Xscale and derivatives to ensure that effects of previous instructions
are seen by following instructios. It predates ARMs barriers.
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according to speedtest.net.
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