powerpc/64s: Don't let DT CPU features set FSCR_DSCR

The device tree CPU features binding includes FSCR bit numbers which
Linux is instructed to set by firmware.

Whether that's a good idea or not, in the case of the DSCR the Linux
implementation has a hard requirement that the FSCR_DSCR bit not be
set by default. We use it to track when a process reads/writes to
DSCR, so it must be clear to begin with.

So if firmware tells us to set FSCR_DSCR we must ignore it.

Currently this does not cause a bug in our DSCR handling because the
value of FSCR that the device tree CPU features code establishes is
only used by swapper. All other tasks use the value hard coded in
init_task.thread.fscr.

However we'd like to fix that in a future commit, at which point this
will become necessary.

Fixes: 5a61ef74 ("powerpc/64s: Support new device tree binding for discovering CPU features")
Cc: stable@vger.kernel.org # v4.12+
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200527145843.2761782-2-mpe@ellerman.id.au

arch/powerpc/kernel/dt_cpu_ftrs.c

@@ -346,6 +346,14 @@ static int __init feat_enable_dscr(struct dt_cpu_feature *f)
 {
 	u64 lpcr;
 
+	/*
+	 * Linux relies on FSCR[DSCR] being clear, so that we can take the
+	 * facility unavailable interrupt and track the task's usage of DSCR.
+	 * See facility_unavailable_exception().
+	 * Clear the bit here so that feat_enable() doesn't set it.
+	 */
+	f->fscr_bit_nr = -1;
+
 	feat_enable(f);
 
 	lpcr = mfspr(SPRN_LPCR);