Platform/RaspberryPi: introduce DebugDualSerialPortLib

On DEBUG builds that use the serial port directly for debug output,
every module reinitializes the UART hardware, through the DebugLib
constructor calling SerialPortInitialize.

This is unnecessary, but usually harmless. However, in cases where this
requires information that is non-trivial to obtain (e.g., the rate of
the clock source feeding the baud clock), it results in a special kind
of dependency hell that can only be fully appreciated by seasoned EDK2
connoisseurs [0].

As a first step towards solving this mess, implement a special version
of the Raspberry Pi dual serial port library that only implements the
SerialPortInitialize() and SerialPortWrite() library functions, and make
the former an empty stub. This makes it only suitable for use by modules
that inherit a dependency on SerialPortLib via DebugLib, and requires us
to ensure that the baud clock is programmed correctly by the SEC phase.

Use this version of the library to satisfy all SerialPortLib dependencies
except the ones in PrePi and in SerialDxe. These will retain the full
version, which is the only one that still consumes PcdSerialClockRate.

[0] There are two distinct problems making this mess almost unsolvable:
  - SerialPortInitialize() is called directly in various places instead
    of relying on constructor ordering, so adding a constructor to a
    SerialPortLib implementation does not help,
  - Constructor ordering resolution in the EDK2 tooling fails to take
    transitive dependencies into account if an intermediate library has
    no constructor it self. For instance, if LibA depends on LibB, which
    depends on LibC, the constructors of LibA and LibC could be called in
    any order if LibB does not have a constructor itself (and fixing this
    breaks all the platforms in the tree)

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Pete Batard <pete@akeo.ie>
Tested-by: Pete Batard <pete@akeo.ie>

1 files changed, 9 insertions, 3 deletions

diff --git a/Platform/RaspberryPi/RPi3/RPi3.dsc b/Platform/RaspberryPi/RPi3/RPi3.dsc
index 563fb891..d7218219 100644
--- a/Platform/RaspberryPi/RPi3/RPi3.dsc
+++ b/Platform/RaspberryPi/RPi3/RPi3.dsc
@@ -127,7 +127,7 @@
   # Dual serial port library

   PL011UartClockLib|ArmPlatformPkg/Library/PL011UartClockLib/PL011UartClockLib.inf

   PL011UartLib|ArmPlatformPkg/Library/PL011UartLib/PL011UartLib.inf

-  SerialPortLib|Platform/RaspberryPi/Library/DualSerialPortLib/DualSerialPortLib.inf

+  SerialPortLib|Platform/RaspberryPi/Library/DualSerialPortLib/DebugDualSerialPortLib.inf

 

   # Cryptographic libraries

   IntrinsicLib|CryptoPkg/Library/IntrinsicLib/IntrinsicLib.inf

@@ -521,7 +521,10 @@
   #

   # PEI Phase modules

   #

-  ArmPlatformPkg/PrePi/PeiUniCore.inf

+  ArmPlatformPkg/PrePi/PeiUniCore.inf {

+    <LibraryClasses>

+      SerialPortLib|Platform/RaspberryPi/Library/DualSerialPortLib/DualSerialPortLib.inf

+  }

 

   #

   # DXE

@@ -569,7 +572,10 @@
   MdeModulePkg/Universal/Console/ConSplitterDxe/ConSplitterDxe.inf

   MdeModulePkg/Universal/Console/GraphicsConsoleDxe/GraphicsConsoleDxe.inf

   MdeModulePkg/Universal/Console/TerminalDxe/TerminalDxe.inf

-  MdeModulePkg/Universal/SerialDxe/SerialDxe.inf

+  MdeModulePkg/Universal/SerialDxe/SerialDxe.inf {

+    <LibraryClasses>

+      SerialPortLib|Platform/RaspberryPi/Library/DualSerialPortLib/DualSerialPortLib.inf

+  }

   Platform/RaspberryPi/Drivers/DisplayDxe/DisplayDxe.inf

 

   MdeModulePkg/Universal/HiiDatabaseDxe/HiiDatabaseDxe.inf