[PATCH edk2-platforms 3/5] Platform/RaspberryPi: fix 16550 divisor calculation logic

["Ard Biesheuvel" <ard.biesheuvel@arm.com>]

The 16550 'miniUART' on the Raspberry Pi gets its input clock from
different sources on RPi3 and RPi3. Fix the logic that derives the
divisor for the 16550 baud clock on the respective platforms.

While at it, make the input clock PCD patchable for RPi3 so we can
manipulate it at runtime in a future patch.

Co-authored-by: Pete Batard <pete@akeo.ie>
Co-authored-by: Andrei Warkentin <andrey.warkentin@gmail.com>
Co-authored-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Signed-off-by: Pete Batard <pete@akeo.ie>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
---
 Platform/RaspberryPi/RPi3/RPi3.dsc                                 |  4 +++-
 Platform/RaspberryPi/RPi4/RPi4.dsc                                 |  2 +-
 Platform/RaspberryPi/Library/DualSerialPortLib/DualSerialPortLib.c | 14 ++++++++------
 3 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/Platform/RaspberryPi/RPi3/RPi3.dsc b/Platform/RaspberryPi/RPi3/RPi3.dsc
index d7218219fc5a..96b27400eef8 100644
--- a/Platform/RaspberryPi/RPi3/RPi3.dsc
+++ b/Platform/RaspberryPi/RPi3/RPi3.dsc
@@ -409,7 +409,6 @@ [PcdsFixedAtBuild.common]
   gArmPlatformTokenSpaceGuid.PL011UartClkInHz|48000000
   gEfiMdeModulePkgTokenSpaceGuid.PcdSerialUseMmio|TRUE
   gEfiMdeModulePkgTokenSpaceGuid.PcdSerialRegisterStride|4
-  gEfiMdeModulePkgTokenSpaceGuid.PcdSerialClockRate|500000000
   gEfiMdeModulePkgTokenSpaceGuid.PcdSerialFifoControl|0x27
   gEfiMdeModulePkgTokenSpaceGuid.PcdSerialExtendedTxFifoSize|8
   gEfiMdePkgTokenSpaceGuid.PcdUartDefaultBaudRate|115200
@@ -441,6 +440,9 @@ [PcdsFixedAtBuild.common]
   gEfiMdeModulePkgTokenSpaceGuid.PcdFirmwareVendor|L"EDK2"
   gEfiMdeModulePkgTokenSpaceGuid.PcdSetNxForStack|TRUE
 
+[PcdsPatchableInModule]
+  gEfiMdeModulePkgTokenSpaceGuid.PcdSerialClockRate|250000000
+
 [PcdsDynamicHii.common.DEFAULT]
 
   #
diff --git a/Platform/RaspberryPi/RPi4/RPi4.dsc b/Platform/RaspberryPi/RPi4/RPi4.dsc
index 4fb015b077e6..5d8bd88d7e34 100644
--- a/Platform/RaspberryPi/RPi4/RPi4.dsc
+++ b/Platform/RaspberryPi/RPi4/RPi4.dsc
@@ -420,7 +420,7 @@ [PcdsFixedAtBuild.common]
   gArmPlatformTokenSpaceGuid.PL011UartClkInHz|48000000
   gEfiMdeModulePkgTokenSpaceGuid.PcdSerialUseMmio|TRUE
   gEfiMdeModulePkgTokenSpaceGuid.PcdSerialRegisterStride|4
-  gEfiMdeModulePkgTokenSpaceGuid.PcdSerialClockRate|500000000
+  gEfiMdeModulePkgTokenSpaceGuid.PcdSerialClockRate|1000000000
   gEfiMdeModulePkgTokenSpaceGuid.PcdSerialFifoControl|0x27
   gEfiMdeModulePkgTokenSpaceGuid.PcdSerialExtendedTxFifoSize|8
   gEfiMdePkgTokenSpaceGuid.PcdUartDefaultBaudRate|115200
diff --git a/Platform/RaspberryPi/Library/DualSerialPortLib/DualSerialPortLib.c b/Platform/RaspberryPi/Library/DualSerialPortLib/DualSerialPortLib.c
index b1d17d3fa04a..5e83bbf022eb 100644
--- a/Platform/RaspberryPi/Library/DualSerialPortLib/DualSerialPortLib.c
+++ b/Platform/RaspberryPi/Library/DualSerialPortLib/DualSerialPortLib.c
@@ -41,18 +41,20 @@ SerialPortGetDivisor (
   // On the Raspberry Pi, the clock to use for the 16650-compatible UART
   // is the base clock divided by the 12.12 fixed point VPU clock divisor.
   //
-  BaseClockRate = (UINT64)PcdGet32 (PcdSerialClockRate) * 4;
+  BaseClockRate = (UINT64)PcdGet32 (PcdSerialClockRate);
+#if (RPI_MODEL == 4)
   Divisor = MmioRead32(BCM2836_CM_BASE + BCM2836_CM_VPU_CLOCK_DIVISOR) & 0xFFFFFF;
   if (Divisor != 0)
     BaseClockRate = (BaseClockRate << 12) / Divisor;
+#endif
 
   //
-  // Now calculate divisor for baud generator
-  //    Ref_Clk_Rate / Baud_Rate / 16
+  // As per the BCM2xxx datasheets:
+  // baudrate = system_clock_freq / (8 * (divisor + 1)).
   //
-  Divisor = (UINT32)BaseClockRate / (SerialBaudRate * 16);
-  if (((UINT32)BaseClockRate % (SerialBaudRate * 16)) >= SerialBaudRate * 8) {
-    Divisor++;
+  Divisor = (UINT32)BaseClockRate / (SerialBaudRate * 8);
+  if (Divisor != 0) {
+    Divisor--;
   }
   return Divisor;
 }
-- 
2.17.1