EmbeddedPkg/Lan9118Dxe: add LAN9118 MMIO wrappers

As described in the LAN9118 datasheet, delays are necessary after some
reads and writes in order to ensure subsequent reads do not see stale
data.

This patch adds helpers to provide these delays automatically, by
performing dummy reads of the BYTE_TEST register (as recommended in the
LAN9118 datasheet). This approach allows the device register file itself
to provide the required delay, avoiding issues with early write
acknowledgement, or re-ordering of MMIO accesses aganist other
instructions (e.g. the delay loop).

Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Ryan Harkin <ryan.harkin@linaro.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

EmbeddedPkg/Drivers/Lan9118Dxe/Lan9118DxeUtil.c

@@ -115,6 +115,54 @@ IndirectMACRead32 (
   return MmioRead32 (LAN9118_MAC_CSR_DATA);
 }
 
+/*
+ * LAN9118 chips have special restrictions on some back-to-back Write/Read or
+ * Read/Read pairs of accesses. After a read or write that changes the state of
+ * the device, there is a period in which stale values may be returned in
+ * response to a read. This period is dependent on the registers accessed.
+ *
+ * We must delay prior reads by this period. This can either be achieved by
+ * timer-based delays, or by performing dummy reads of the BYTE_TEST register,
+ * for which the recommended number of reads is described in the LAN9118 data
+ * sheet. This is required in addition to any memory barriers.
+ *
+ * This function performs a number of dummy reads of the BYTE_TEST register, as
+ * a building block for the above.
+ */
+VOID
+WaitDummyReads (
+  UINTN Count
+  )
+{
+       while (Count--)
+         MmioRead32(LAN9118_BYTE_TEST);
+}
+
+UINT32
+Lan9118RawMmioRead32(
+  UINTN Address,
+  UINTN Delay
+  )
+{
+  UINT32 Value;
+
+  Value = MmioRead32(Address);
+  WaitDummyReads(Delay);
+  return Value;
+}
+
+UINT32
+Lan9118RawMmioWrite32(
+  UINTN Address,
+  UINT32 Value,
+  UINTN Delay
+  )
+{
+  MmioWrite32(Address, Value);
+  WaitDummyReads(Delay);
+  return Value;
+}
+
 // Function to write to MAC indirect registers
 UINT32
 IndirectMACWrite32 (