Commit 7662abf4db94 ("net: phy: sfp: Add support for SMBus module access")
added support for SMBus-only controllers for module access. However,
this is restricted to single-byte accesses and has the implication that
hwmon is disabled (due to missing atomicity of 16-bit accesses) and
warnings are printed.

There are probably a lot of SMBus-only I2C controllers out in the wild
which support block reads. Right now, they don't work with SFP modules.
This applies - amongst others - to I2C/SMBus-only controllers in Realtek
longan and mango SoCs.

Downstream in OpenWrt, a patch similar to the abovementioned patch is
used for current LTS kernel 6.12. However, this uses byte-access for all
kinds of access and thus disregards the atomicity for wider access.

Introduce read/write SMBus I2C block operations to support SMBus-only
controllers with appropriate support for block read/write. Those
operations are used for all accesses if supported, otherwise the
single-byte operations will be used. With block reads, atomicity for
16-bit reads as required by hwmon is preserved and thus, hwmon can be
used.

The implementation requires the I2C_FUNC_SMBUS_I2C_BLOCK to be
supported as it relies on reading a pre-defined amount of bytes.
This isn't intended by the official SMBus Block Read but supported by
several I2C controllers/drivers.

Support for word access is not implemented due to issues regarding
endianness.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
---
 drivers/net/phy/sfp.c | 77 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 75 insertions(+), 2 deletions(-)

diff --git a/drivers/net/phy/sfp.c b/drivers/net/phy/sfp.c
index 6166e9196364..c5019fb682a3 100644
--- a/drivers/net/phy/sfp.c
+++ b/drivers/net/phy/sfp.c
@@ -744,6 +744,35 @@ static int sfp_smbus_byte_read(struct sfp *sfp, bool a2, u8 dev_addr,
 	return data - (u8 *)buf;
 }
 
+static int sfp_smbus_block_read(struct sfp *sfp, bool a2, u8 dev_addr,
+				void *buf, size_t len)
+{
+	size_t block_size = sfp->i2c_block_size;
+	union i2c_smbus_data smbus_data;
+	u8 bus_addr = a2 ? 0x51 : 0x50;
+	u8 *data = buf;
+	u8 this_len;
+	int ret;
+
+	while (len) {
+		this_len = min(len, block_size);
+
+		smbus_data.block[0] = this_len;
+		ret = i2c_smbus_xfer(sfp->i2c, bus_addr, 0,
+				     I2C_SMBUS_READ, dev_addr,
+				     I2C_SMBUS_I2C_BLOCK_DATA, &smbus_data);
+		if (ret < 0)
+			return ret;
+
+		memcpy(data, &smbus_data.block[1], this_len);
+		len -= this_len;
+		data += this_len;
+		dev_addr += this_len;
+	}
+
+	return data - (u8 *)buf;
+}
+
 static int sfp_smbus_byte_write(struct sfp *sfp, bool a2, u8 dev_addr,
 				void *buf, size_t len)
 {
@@ -768,23 +797,67 @@ static int sfp_smbus_byte_write(struct sfp *sfp, bool a2, u8 dev_addr,
 	return 0;
 }
 
+static int sfp_smbus_block_write(struct sfp *sfp, bool a2, u8 dev_addr,
+				 void *buf, size_t len)
+{
+	size_t block_size = sfp->i2c_block_size;
+	union i2c_smbus_data smbus_data;
+	u8 bus_addr = a2 ? 0x51 : 0x50;
+	u8 *data = buf;
+	u8 this_len;
+	int ret;
+
+	while (len) {
+		this_len = min(len, block_size);
+
+		smbus_data.block[0] = this_len;
+		memcpy(&smbus_data.block[1], data, this_len);
+		ret = i2c_smbus_xfer(sfp->i2c, bus_addr, 0,
+				     I2C_SMBUS_WRITE, dev_addr,
+				     I2C_SMBUS_I2C_BLOCK_DATA, &smbus_data);
+		if (ret)
+			return ret;
+
+		len -= this_len;
+		data += this_len;
+		dev_addr += this_len;
+	}
+
+	return 0;
+}
+
 static int sfp_i2c_configure(struct sfp *sfp, struct i2c_adapter *i2c)
 {
+	size_t max_block_size;
+
 	sfp->i2c = i2c;
 
 	if (i2c_check_functionality(i2c, I2C_FUNC_I2C)) {
 		sfp->read = sfp_i2c_read;
 		sfp->write = sfp_i2c_write;
-		sfp->i2c_max_block_size = SFP_EEPROM_BLOCK_SIZE;
+		max_block_size = SFP_EEPROM_BLOCK_SIZE;
+	} else if (i2c_check_functionality(i2c, I2C_FUNC_SMBUS_I2C_BLOCK)) {
+		sfp->read = sfp_smbus_block_read;
+		sfp->write = sfp_smbus_block_write;
+
+		max_block_size = SFP_EEPROM_BLOCK_SIZE;
+		if (i2c->quirks && i2c->quirks->max_read_len)
+			max_block_size = min(max_block_size,
+					     i2c->quirks->max_read_len);
+		if (i2c->quirks && i2c->quirks->max_write_len)
+			max_block_size = min(max_block_size,
+					     i2c->quirks->max_write_len);
+
 	} else if (i2c_check_functionality(i2c, I2C_FUNC_SMBUS_BYTE_DATA)) {
 		sfp->read = sfp_smbus_byte_read;
 		sfp->write = sfp_smbus_byte_write;
-		sfp->i2c_max_block_size = 1;
+		max_block_size = 1;
 	} else {
 		sfp->i2c = NULL;
 		return -EINVAL;
 	}
 
+	sfp->i2c_max_block_size = max_block_size;
 	return 0;
 }
 

base-commit: 3387a7ad478b46970ae8254049167d166e398aeb
-- 
2.48.1