[PATCH] rtc: gamecube: Fix supported time range in gamecube_rtc_probe()

[Thorsten Blum <blum@kernel.org>]

gamecube_rtc_read_time() adds the SRAM bias to the 32-bit RTC counter.
However, gamecube_rtc_probe() sets the supported time range from 0 to
U32_MAX without taking the bias into account.

gamecube_rtc_set_time() subtracts the bias from the timestamp, and if
the timestamp is smaller than the bias, the result is negative and wraps
when passed to regmap_write(), storing the wrong time.

Add the bias to the supported range to match gamecube_rtc_read_time()
and gamecube_rtc_set_time(). Setting a time below the bias now returns
-ERANGE. This also allows setting dates beyond the 2106 limit. Update
the comment accordingly.

Fixes: 86559400b3ef ("rtc: gamecube: Add a RTC driver for the GameCube, Wii and Wii U")
Cc: stable@vger.kernel.org
Signed-off-by: Thorsten Blum <blum@kernel.org>
---
Not tested on hardware.

 drivers/rtc/rtc-gamecube.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/drivers/rtc/rtc-gamecube.c b/drivers/rtc/rtc-gamecube.c
index 889028cecf4a..276e1f889d34 100644
--- a/drivers/rtc/rtc-gamecube.c
+++ b/drivers/rtc/rtc-gamecube.c
@@ -347,12 +347,9 @@ static int gamecube_rtc_probe(struct platform_device *pdev)
 	if (IS_ERR(rtc))
 		return PTR_ERR(rtc);
 
-	/* We can represent further than that, but it depends on the stored
-	 * bias and we can’t modify it persistently on all supported consoles,
-	 * so here we pretend to be limited to 2106.
-	 */
-	rtc->range_min = 0;
-	rtc->range_max = U32_MAX;
+	/* Shift the range since the RTC counter is offset by the SRAM bias. */
+	rtc->range_min = d->rtc_bias;
+	rtc->range_max = (timeu64_t)d->rtc_bias + U32_MAX;
 	rtc->ops = &gamecube_rtc_ops;
 
 	return devm_rtc_register_device(rtc);