[RFC PATCH 3/4] powerpc/64s: Save FSCR to init_task.thread.fscr after feature init

[Michael Ellerman <mpe@ellerman.id.au>]

At boot the FSCR is initialised via one of two paths. On most systems
it's set to a hard coded value in __init_FSCR().

On newer skiboot systems we use the device tree CPU features binding,
where firmware can tell Linux what bits to set in FSCR (and HFSCR).

In both cases the value that's configured at boot is not propagated
into the init_task.thread.fscr value prior to the initial fork of init
(pid 1), which means the value is not used by any processes other than
swapper (the idle task).

For the __init_FSCR() case this is OK, because the value in
init_task.thread.fscr is initialised to something sensible. However it
does mean that the value set in __init_FSCR() is not used other than
for swapper, which is odd and confusing.

The bigger problem is for the device tree CPU features case it
prevents firmware from setting (or clearing) FSCR bits for use by user
space. This means all existing kernels can not have features
enabled/disabled by firmware if those features require
setting/clearing FSCR bits.

We can handle both cases by saving the FSCR value into
init_task.thread.fscr after we have initialised it at boot. This fixes
the bug for device tree CPU features, and will allow us to simplify
the initialisation for the __init_FSCR() case in a future patch.

Fixes: 5a61ef74f269 ("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>
---
 arch/powerpc/kernel/prom.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index 1dcf0e214a22..a74bfd09cb38 100644
--- a/arch/powerpc/kernel/prom.c
+++ b/arch/powerpc/kernel/prom.c
@@ -685,6 +685,23 @@ static void __init tm_init(void)
 static void tm_init(void) { }
 #endif /* CONFIG_PPC_TRANSACTIONAL_MEM */
 
+#ifdef CONFIG_PPC64
+static void __init save_fscr_to_task(void)
+{
+	/*
+	 * Ensure the init_task (pid 0, aka swapper) uses the value of FSCR we
+	 * have configured via the device tree features or via __init_FSCR().
+	 * That value will then be propagated to pid 1 (init) and all future
+	 * processes.
+	 */
+	if (early_cpu_has_feature(CPU_FTR_ARCH_207S))
+		init_task.thread.fscr = mfspr(SPRN_FSCR);
+}
+#else
+static inline void save_fscr_to_task(void) {};
+#endif
+
+
 void __init early_init_devtree(void *params)
 {
 	phys_addr_t limit;
@@ -773,6 +790,8 @@ void __init early_init_devtree(void *params)
 		BUG();
 	}
 
+	save_fscr_to_task();
+
 #if defined(CONFIG_SMP) && defined(CONFIG_PPC64)
 	/* We'll later wait for secondaries to check in; there are
 	 * NCPUS-1 non-boot CPUs  :-)
-- 
2.25.1