i2c-imc-skylake: implement Receive Byte and Send Byte

The engine encodes an address-only transaction in command word bit 18,
which is what SMBus Receive Byte and Send Byte need and what was missing
for ee1004 to bind.

The bit is not documented for the PCU function. It was located by
structure rather than by sweeping: SMBCMD on the iMC channel function
(datasheet ref. 614073 section 3.1.8) places PNTR_SEL immediately above
WORD_ACCESS, and word access here is bit 17. Bit 17 had itself been
unknown until it turned out to be word access, and an earlier sweep of
this register covered bits 31:20 only, so 18 was untried rather than
already ruled out.

Validated against the device rather than against the register. An EEPROM
auto-increments its pointer after a read, so priming the pointer and
repeating the read with the bit set has a predicted answer: the next
byte, not the one named. Eight consecutive reads returned SPD bytes 0x01
through 0x08 in order, matching an ordinary dump. Send Byte was confirmed
on the DDR4 page-select addresses, which hold a latch and store no data,
so the page state is itself the readout.

Result: ee1004 binds on all four SPD addresses and decode-dimms decodes
all four modules with both CRCs valid on each. The part number comes off
page 1, so the page select runs end to end. The four module serial
numbers are unchanged from the baseline recorded before this series.

SMBus Quick stays unimplemented. Beyond having no obvious encoding, a
write into the 0x30-0x37 range is how an EE1004 SPD is write-protected,
permanently on many modules, so an adapter on the memory bus should not
advertise a probe that writes. i2cdetect scans in read mode.

Also fix test-smoke.sh resolving the module through modinfo, which
silently tested the installed DKMS build instead of the one just
compiled. It now prefers the local .ko and prints which it loaded.

Smoke suite: 19 passed, 0 failed. checkpatch --strict 0/0/0, in-tree W=1
and Sparse clean.

test-smoke.sh

@@ -19,7 +19,23 @@ else
 	fi
 fi
 
-MODULE_PATH=${MODULE_PATH:-$(modinfo -n i2c-imc-skylake)}
+# Prefer the module built in this directory. Falling straight through to
+# modinfo would silently test whichever build is installed, which on a machine
+# with a DKMS package is not the one just compiled.
+_default_module()
+{
+	local local_build
+
+	local_build=$(dirname "$(readlink -f "$0")")/i2c-imc-skylake.ko
+	if [[ -r "$local_build" ]]; then
+		printf '%s\n' "$local_build"
+	else
+		modinfo -n i2c-imc-skylake
+	fi
+}
+
+MODULE_PATH=${MODULE_PATH:-$(_default_module)}
+echo "testing module: $MODULE_PATH"
 if [[ ! -r "$MODULE_PATH" ]]; then
 	echo "Module not found: $MODULE_PATH" >&2
 	exit 2
@@ -118,6 +134,46 @@ _word_test()
 	((word == expected))
 }
 
+# SMBus Receive Byte reads from the device's own pointer, so on an EEPROM a
+# sequence of them walks forward through the array. Prime the pointer with an
+# ordinary read, then check the next reads reproduce an ordinary dump of the
+# following bytes. A transaction that still named a register would return the
+# same byte every time.
+_receive_byte_test()
+{
+	local bus=$1 address=$2
+	local offset expected got
+
+	"${SUDO[@]}" i2cget -y "$bus" "$address" 0x00 >/dev/null 2>&1 || return 1
+	for offset in 0x01 0x02 0x03 0x04; do
+		got=$("${SUDO[@]}" i2cget -y "$bus" "$address" 2>/dev/null) || return 1
+		expected=$("${SUDO[@]}" i2cget -y "$bus" "$address" "$offset" 2>/dev/null) || return 1
+		((got == expected)) || return 1
+		# The explicit read above moved the pointer to $offset + 1, which
+		# is where the next Receive Byte should pick up.
+	done
+}
+
+# SMBus Send Byte against the DDR4 page-select addresses. Selecting page 1 puts
+# the upper half of the SPD in the 256-byte window, so byte 0x00 stops being
+# the JEDEC 0x23; selecting page 0 brings it back. The page state is the
+# readout, so the test verifies itself, and it ends on page 0 either way.
+_send_byte_test()
+{
+	local bus=$1 address=$2
+	local base flipped restored
+
+	base=$("${SUDO[@]}" i2cget -y "$bus" "$address" 0x00 2>/dev/null) || return 1
+
+	"${SUDO[@]}" i2cset -y "$bus" 0x37 0x00 c 2>/dev/null
+	flipped=$("${SUDO[@]}" i2cget -y "$bus" "$address" 0x00 2>/dev/null)
+
+	"${SUDO[@]}" i2cset -y "$bus" 0x36 0x00 c 2>/dev/null
+	restored=$("${SUDO[@]}" i2cget -y "$bus" "$address" 0x00 2>/dev/null)
+
+	[[ "$flipped" != "$base" && "$restored" == "$base" ]]
+}
+
 _nack_test()
 {
 	local bus=$1
@@ -169,6 +225,26 @@ if [[ -n "$CH1_ADDR" ]]; then
 	_word_test "$CH1" "$CH1_ADDR" && _ok "WORD_DATA byte order on ch1" || _fail "WORD_DATA test on ch1"
 
 fi
+if [[ -n "$CH0_ADDR" ]]; then
+	_receive_byte_test "$CH0" "$CH0_ADDR" &&
+		_ok "Receive Byte follows the EEPROM pointer on ch0" ||
+		_fail "Receive Byte test on ch0"
+fi
+if [[ -n "$CH1_ADDR" ]]; then
+	_receive_byte_test "$CH1" "$CH1_ADDR" &&
+		_ok "Receive Byte follows the EEPROM pointer on ch1" ||
+		_fail "Receive Byte test on ch1"
+fi
+if [[ -n "$CH0_ADDR" ]]; then
+	_send_byte_test "$CH0" "$CH0_ADDR" &&
+		_ok "Send Byte selects the SPD page on ch0" ||
+		_fail "Send Byte test on ch0"
+fi
+if [[ -n "$CH1_ADDR" ]]; then
+	_send_byte_test "$CH1" "$CH1_ADDR" &&
+		_ok "Send Byte selects the SPD page on ch1" ||
+		_fail "Send Byte test on ch1"
+fi
 if [[ -n "$CH0" ]]; then
 	_nack_test "$CH0" && _ok "absent-device ENXIO on ch0" || _fail "expected ENXIO at address 0x77 on ch0"
 fi