rtc: mc146818-lib: Fix the AltCentury for AMD platforms

BugLink: https://bugs.launchpad.net/bugs/1981375
[ Upstream commit 3ae8fd41573af4fb3a490c9ed947fc936ba87190 ]

Setting the century forward has been failing on AMD platforms.
There was a previous attempt at fixing this for family 0x17 as part of
commit 7ad295d5196a ("rtc: Fix the AltCentury value on AMD/Hygon
platform") but this was later reverted due to some problems reported
that appeared to stem from an FW bug on a family 0x17 desktop system.

The same comments mentioned in the previous commit continue to apply
to the newer platforms as well.

```
MC146818 driver use function mc146818_set_time() to set register
RTC_FREQ_SELECT(RTC_REG_A)'s bit4-bit6 field which means divider stage
reset value on Intel platform to 0x7.

While AMD/Hygon RTC_REG_A(0Ah)'s bit4 is defined as DV0 [Reference]:
DV0 = 0 selects Bank 0, DV0 = 1 selects Bank 1. Bit5-bit6 is defined
as reserved.

DV0 is set to 1, it will select Bank 1, which will disable AltCentury
register(0x32) access. As UEFI pass acpi_gbl_FADT.century 0x32
(AltCentury), the CMOS write will be failed on code:
CMOS_WRITE(century, acpi_gbl_FADT.century).

Correct RTC_REG_A bank select bit(DV0) to 0 on AMD/Hygon CPUs, it will
enable AltCentury(0x32) register writing and finally setup century as
expected.
```

However in closer examination the change previously submitted was also
modifying bits 5 & 6 which are declared reserved in the AMD documentation.
So instead modify just the DV0 bank selection bit.

Being cognizant that there was a failure reported before, split the code
change out to a static function that can also be used for exclusions if
any regressions such as Mikhail's pop up again.

Cc: Jinke Fan <fanjinke@hygon.cn>
Cc: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Link: https://lore.kernel.org/all/CABXGCsMLob0DC25JS8wwAYydnDoHBSoMh2_YLPfqm3TTvDE-Zw@mail.gmail.com/
Link: https://www.amd.com/system/files/TechDocs/51192_Bolton_FCH_RRG.pdf
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220111225750.1699-1-mario.limonciello@amd.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/drivers/rtc/rtc-mc146818-lib.c b/drivers/rtc/rtc-mc146818-lib.c
index 8abac672f3cbd0efaceeae1e30cd399aff5c6b07..f3f5a87fe376e02505d783287436f00ea49d4797 100644 (file)
--- a/drivers/rtc/rtc-mc146818-lib.c
+++ b/drivers/rtc/rtc-mc146818-lib.c
@@ -146,6 +146,17 @@ again:
 }
 EXPORT_SYMBOL_GPL(mc146818_get_time);
 
+/* AMD systems don't allow access to AltCentury with DV1 */
+static bool apply_amd_register_a_behavior(void)
+{
+#ifdef CONFIG_X86
+       if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
+           boot_cpu_data.x86_vendor == X86_VENDOR_HYGON)
+               return true;
+#endif
+       return false;
+}
+
 /* Set the current date and time in the real time clock. */
 int mc146818_set_time(struct rtc_time *time)
 {
@@ -219,7 +230,10 @@ int mc146818_set_time(struct rtc_time *time)
        save_control = CMOS_READ(RTC_CONTROL);
        CMOS_WRITE((save_control|RTC_SET), RTC_CONTROL);
        save_freq_select = CMOS_READ(RTC_FREQ_SELECT);
-       CMOS_WRITE((save_freq_select|RTC_DIV_RESET2), RTC_FREQ_SELECT);
+       if (apply_amd_register_a_behavior())
+               CMOS_WRITE((save_freq_select & ~RTC_AMD_BANK_SELECT), RTC_FREQ_SELECT);
+       else
+               CMOS_WRITE((save_freq_select|RTC_DIV_RESET2), RTC_FREQ_SELECT);
 
 #ifdef CONFIG_MACH_DECSTATION
        CMOS_WRITE(real_yrs, RTC_DEC_YEAR);

diff --git a/include/linux/mc146818rtc.h b/include/linux/mc146818rtc.h
index 37c74e25f53ddf8ec1b57cddcf241e715426d32d..3038124c6115495c19c7b4626af5e112f635f711 100644 (file)
--- a/include/linux/mc146818rtc.h
+++ b/include/linux/mc146818rtc.h
@@ -86,6 +86,8 @@ struct cmos_rtc_board_info {
    /* 2 values for divider stage reset, others for "testing purposes only" */
 #  define RTC_DIV_RESET1       0x60
 #  define RTC_DIV_RESET2       0x70
+   /* In AMD BKDG bit 5 and 6 are reserved, bit 4 is for select dv0 bank */
+#  define RTC_AMD_BANK_SELECT  0x10
   /* Periodic intr. / Square wave rate select. 0=none, 1=32.8kHz,... 15=2Hz */
 # define RTC_RATE_SELECT       0x0F