chromeec: support reading long battery strings

The Chrome EC currently supports two ways to read battery strings on
ACPI platforms:

 * Read up to 8 bytes from EC shared memory BMFG, BMOD, ...
 * Send a EC_CMD_BATTERY_GET_STATIC host command and read strings from
   the response. This is assumed to be exclusively controlled by the OS,
   because host commands' use of buffers is prone to race conditions.

To support readout of longer strings via ACPI mechanisms, this change
adds support for EC_ACPI_MEM_STRINGS_FIFO (https://crrev.com/c/5581473)
and allows ACPI firmware to read strings of arbitrary length (currently
limited to 64 characters in the implementation) from the EC and to
determine whether this function is supported by the EC (falling back to
shared memory if not).

BUG=b:339171261
TEST=on yaviks, the EC console logs FIFO readout messages when used in
     ACPI and correct strings are shown in the OS. If EC support is
     removed, correct strings are still shown in the OS.
BRANCH=nissa

Change-Id: Ia29cacb7d86402490f9ac458f0be50e3f2192b04
Signed-off-by: Peter Marheine <pmarheine@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82775
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Subrata Banik <subratabanik@google.com>

src/ec/google/chromeec/acpi/battery.asl

@@ -54,6 +54,91 @@ Method (BSTA, 1, Serialized)
 	Return (Local0)
 }
 
+// Cached flag for BSRF FIFO readout support from EC.
+Name(BRSS, 0xff)
+
+// Read extended battery strings from the selected battery.
+//   Arg0 = string index
+//
+// If supported by the EC, strings of arbitrary length are read using the
+// FIFO readout method. Otherwise short (no more than 8 bytes) strings are
+// read from the EC shared memory map. The desired battery index should be
+// selected with BTSW before calling this method.
+//
+// Currently supported string indexes:
+//  * 1 = EC_ACPI_MEM_STRINGS_FIFO_ID_BATTERY_MODEL: battery model name,
+//        equivalent to BMOD.
+//  * 2 = EC_ACPI_MEM_STRINGS_FIFO_ID_BATTERY_SERIAL: battery serial number,
+//        equivalent to BSER.
+//  * 3 = EC_ACPI_MEM_STRINGS_FIFO_ID_BATTERY_MANUFACTURER: battery
+//        manufacturer's name, equivalent to BMFG.
+//
+// These are assumed to be supported if the EC reports at least version 1 of
+// string readout (it returns an integer greater than 0 and less than 255 when
+// reading FIFO index 0). Future strings may require higher FIFO versions.
+Method(BRSX, 1, Serialized)
+{
+	// It doesn't make sense to read the FIFO support indicator.
+	if (Arg0 == 0)
+	{
+		Return ("")
+	}
+
+	If (BRSS == 0xff)
+	{
+		// Write 0 to BSRF to read back a support indicator; nonzero and
+		// non-0xFF if FIFO readout is supported, assuming minimum v1 support
+		// for strings 1 through 3.
+		BSRF = 0
+		BRSS = BSRF
+
+		// 0xff readback also means no support for FIFO readout, when the EC
+		// doesn't even know what this command is.
+		if (BRSS == 0xff)
+		{
+			BRSS = 0
+		}
+	}
+
+	// If FIFO readout through BSRF is not supported, fall back to reading
+	// the short strings in EMEM.
+	If (BRSS == 0)
+	{
+		If (Arg0 == 1)
+		{
+			Return (ToString (Concatenate (BMOD, 0)))
+		}
+		ElseIf (Arg0 == 2)
+		{
+			Return (ToString (Concatenate (BSER, 0)))
+		}
+		ElseIf (Arg0 == 3)
+		{
+			Return (ToString (Concatenate (BMFG, 0)))
+		}
+		Else
+		{
+			Return ("")
+		}
+	}
+
+	// Select requested parameter to read
+	BSRF = Arg0
+
+	// Read to end of string, or up to a reasonable maximum length. Reads of
+	// BSRF consume bytes from the FIFO, so take care to read it only once
+	// per byte of data.
+	Local0 = ""
+	Local1 = BSRF
+	While (Local1 != 0 && SizeOf (Local0) < 32)
+	{
+		Local0 = Concatenate (Local0, ToString (Local1))
+		Local1 = BSRF
+	}
+
+	Return (Local0)
+}
+
 // _BIF implementation.
 //   Arg0 = battery index
 //   Arg1 = PBIF
@@ -86,9 +171,9 @@ Method (BBIF, 2, Serialized)
 	Arg1[6] = Local2
 
 	// Get battery info from mainboard
-	Arg1[9] = ToString(Concatenate(BMOD, 0x00))
-	Arg1[10] = ToString(Concatenate(BSER, 0x00))
-	Arg1[12] = ToString(Concatenate(BMFG, 0x00))
+	Arg1[9] = BRSX (1)
+	Arg1[10] = BRSX (2)
+	Arg1[12] = BRSX (3)
 
 	Release (^BATM)
 	Return (Arg1)
@@ -129,9 +214,9 @@ Method (BBIX, 2, Serialized)
 	Arg1[8] = BTCC
 
 	// Get battery info from mainboard
-	Arg1[16] = ToString(Concatenate(BMOD, 0x00))
-	Arg1[17] = ToString(Concatenate(BSER, 0x00))
-	Arg1[19] = ToString(Concatenate(BMFG, 0x00))
+	Arg1[16] = BRSX (1)
+	Arg1[17] = BRSX (2)
+	Arg1[19] = BRSX (3)
 
 	Release (^BATM)
 	Return (Arg1)

src/ec/google/chromeec/acpi/ec.asl

@@ -100,6 +100,7 @@ Device (EC0)
 		USPP, 8,	// USB Port Power
 		RFWU, 8,	// Retimer Firmware Update
 		PBOK, 8,	// Power source change count from dptf
+		BSRF, 8,	// Battery string readout FIFO
 	}
 
 #if CONFIG(EC_GOOGLE_CHROMEEC_ACPI_MEMMAP)