powerpc/fsl-booke: Work around erratum A-006958

Erratum A-006598 says that 64-bit mftb is not atomic -- it's subject
to a similar race condition as doing mftbu/mftbl on 32-bit.  The lower
half of timebase is updated before the upper half; thus, we can share
the workaround for a similar bug on Cell.  This workaround involves
looping if the lower half of timebase is zero, thus avoiding the need
for a scratch register (other than CR0).  This workaround must be
avoided when the timebase is frozen, such as during the timebase sync
code.

This deals with kernel and vdso accesses, but other userspace accesses
will of course need to be fixed elsewhere.

Signed-off-by: Scott Wood <scottwood@freescale.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/arch/powerpc/platforms/85xx/smp.c b/arch/powerpc/platforms/85xx/smp.c
index ea9c6269ead0..ea7e62910891 100644
--- a/arch/powerpc/platforms/85xx/smp.c
+++ b/arch/powerpc/platforms/85xx/smp.c
@@ -69,7 +69,30 @@ static void mpc85xx_give_timebase(void)
 	tb_req = 0;
 
 	mpc85xx_timebase_freeze(1);
+#ifdef CONFIG_PPC64
+	/*
+	 * e5500/e6500 have a workaround for erratum A-006958 in place
+	 * that will reread the timebase until TBL is non-zero.
+	 * That would be a bad thing when the timebase is frozen.
+	 *
+	 * Thus, we read it manually, and instead of checking that
+	 * TBL is non-zero, we ensure that TB does not change.  We don't
+	 * do that for the main mftb implementation, because it requires
+	 * a scratch register
+	 */
+	{
+		u64 prev;
+
+		asm volatile("mftb %0" : "=r" (timebase));
+
+		do {
+			prev = timebase;
+			asm volatile("mftb %0" : "=r" (timebase));
+		} while (prev != timebase);
+	}
+#else
 	timebase = get_tb();
+#endif
 	mb();
 	tb_valid = 1;