commit 179a502f8c46 ("rtc: snvs: add Freescale rtc-snvs driver") introduces
the SNVS RTC driver with a function snvs_rtc_enable().

snvs_rtc_enable() can return an error on the enable path however this
driver does not currently trap that failure on the probe() path and
consequently if enabling the RTC fails we encounter a later error spinning
forever in rtc_write_sync_lp().

[   36.093481] [<c010d630>] (__irq_svc) from [<c0c2e9ec>] 
(_raw_spin_unlock_irqrestore+0x34/0x44)
[   36.102122] [<c0c2e9ec>] (_raw_spin_unlock_irqrestore) from [<c072e32c>] 
(regmap_read+0x4c/0x5c)
[   36.110938] [<c072e32c>] (regmap_read) from [<c085d0f4>] 
(rtc_write_sync_lp+0x6c/0x98)
[   36.118881] [<c085d0f4>] (rtc_write_sync_lp) from [<c085d160>] 
(snvs_rtc_alarm_irq_enable+0x40/0x4c)
[   36.128041] [<c085d160>] (snvs_rtc_alarm_irq_enable) from [<c08567b4>] 
(rtc_timer_do_work+0xd8/0x1a8)
[   36.137291] [<c08567b4>] (rtc_timer_do_work) from [<c01441b8>] 
(process_one_work+0x28c/0x76c)
[   36.145840] [<c01441b8>] (process_one_work) from [<c01446cc>] 
(worker_thread+0x34/0x58c)
[   36.153961] [<c01446cc>] (worker_thread) from [<c014aee4>] 
(kthread+0x138/0x150)
[   36.161388] [<c014aee4>] (kthread) from [<c0107e14>] 
(ret_from_fork+0x14/0x20)
[   36.168635] rcu_sched kthread starved for 2602 jiffies! g496 c495 f0x2 
RCU_GP_WAIT_FQS(3) ->state=0x0 ->cpu=0
[   36.178564] rcu_sched       R  running task        0     8      2 0x00000000
[   36.185664] [<c0c288b0>] (__schedule) from [<c0c29134>] (schedule+0x3c/0xa0)
[   36.192739] [<c0c29134>] (schedule) from [<c0c2db80>] 
(schedule_timeout+0x78/0x4e0)
[   36.200422] [<c0c2db80>] (schedule_timeout) from [<c01a7ab0>] 
(rcu_gp_kthread+0x648/0x1864)
[   36.208800] [<c01a7ab0>] (rcu_gp_kthread) from [<c014aee4>] 
(kthread+0x138/0x150)
[   36.216309] [<c014aee4>] (kthread) from [<c0107e14>] 
(ret_from_fork+0x14/0x20)

This patch fixes by parsing the result of rtc_write_sync_lp() and
propagating both in the probe and elsewhere. If the RTC doesn't start we
don't proceed loading the driver and don't get into this loop mess later
on.

Fixes: 179a502f8c46 ("rtc: snvs: add Freescale rtc-snvs driver")

Signed-off-by: Bryan O'Donoghue <pure.lo...@nexus-software.ie>
Cc: a.zu...@towertech.it
Cc: alexandre.bell...@free-electrons.com
Cc: linux-...@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: <sta...@vger.kernel.org> # v3.16+
---
 drivers/rtc/rtc-snvs.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/rtc/rtc-snvs.c b/drivers/rtc/rtc-snvs.c
index d8ef9e0..9af591d 100644
--- a/drivers/rtc/rtc-snvs.c
+++ b/drivers/rtc/rtc-snvs.c
@@ -132,20 +132,23 @@ static int snvs_rtc_set_time(struct device *dev, struct 
rtc_time *tm)
 {
        struct snvs_rtc_data *data = dev_get_drvdata(dev);
        unsigned long time;
+       int ret;
 
        rtc_tm_to_time(tm, &time);
 
        /* Disable RTC first */
-       snvs_rtc_enable(data, false);
+       ret = snvs_rtc_enable(data, false);
+       if (ret)
+               return ret;
 
        /* Write 32-bit time to 47-bit timer, leaving 15 LSBs blank */
        regmap_write(data->regmap, data->offset + SNVS_LPSRTCLR, time << 
CNTR_TO_SECS_SH);
        regmap_write(data->regmap, data->offset + SNVS_LPSRTCMR, time >> (32 - 
CNTR_TO_SECS_SH));
 
        /* Enable RTC again */
-       snvs_rtc_enable(data, true);
+       ret = snvs_rtc_enable(data, true);
 
-       return 0;
+       return ret;
 }
 
 static int snvs_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alrm)
@@ -288,7 +291,11 @@ static int snvs_rtc_probe(struct platform_device *pdev)
        regmap_write(data->regmap, data->offset + SNVS_LPSR, 0xffffffff);
 
        /* Enable RTC */
-       snvs_rtc_enable(data, true);
+       ret = snvs_rtc_enable(data, true);
+       if (ret) {
+               dev_err(&pdev->dev, "failed to enable rtc %d\n", ret);
+               goto error_rtc_device_register;
+       }
 
        device_init_wakeup(&pdev->dev, true);
 
-- 
2.7.4

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