Re: [PATCH 1/3] hwmon: max31827: Add PEC support

Guenter Roeck <linux@xxxxxxxxxxxx> · Mon, 8 Jan 2024 09:42:08 -0800

On 12/19/23 17:49, Matyas, Daniel wrote:

-----Original Message-----
From: Guenter Roeck <groeck7@xxxxxxxxx> On Behalf Of Guenter Roeck
Sent: Monday, December 18, 2023 9:01 PM
To: Matyas, Daniel <Daniel.Matyas@xxxxxxxxxx>
Cc: Jean Delvare <jdelvare@xxxxxxxx>; Rob Herring
<robh+dt@xxxxxxxxxx>; Krzysztof Kozlowski
<krzysztof.kozlowski+dt@xxxxxxxxxx>; Conor Dooley
<conor+dt@xxxxxxxxxx>; Jonathan Corbet <corbet@xxxxxxx>; linux-
hwmon@xxxxxxxxxxxxxxx; devicetree@xxxxxxxxxxxxxxx; linux-
kernel@xxxxxxxxxxxxxxx; linux-doc@xxxxxxxxxxxxxxx
Subject: Re: [PATCH 1/3] hwmon: max31827: Add PEC support

[External]

On 12/18/23 09:59, Matyas, Daniel wrote:

----------------------------------------------------------------------
----------------------------------------------------------------------
----------------------------------------------------------------------
----------------------------------------------------------------------
----------------------------------------------------------------------
----------------------------------------------------------------------
----------------------------------------------------------------------
----------------------------------------------------------------------
----------------------------------------------------------------------
----------------------------------------------------------------------
----------------------------------------------------------------------
----------------------------------------------------------------------
----------------------------------------------------------------------
----------------------------------------------------------------------
----------
*Von:* Guenter Roeck <groeck7@xxxxxxxxx> im Auftrag von Guenter
Roeck
<linux@xxxxxxxxxxxx>
*Gesendet:* Montag, Dezember 18, 2023 6:26:57 nachm.
*An:* Matyas, Daniel <Daniel.Matyas@xxxxxxxxxx>
*Cc:* Jean Delvare <jdelvare@xxxxxxxx>; Rob Herring
<robh+dt@xxxxxxxxxx>; Krzysztof Kozlowski
<krzysztof.kozlowski+dt@xxxxxxxxxx>; Conor Dooley
<conor+dt@xxxxxxxxxx>; Jonathan Corbet <corbet@xxxxxxx>;
linux-hwmon@xxxxxxxxxxxxxxx <linux-hwmon@xxxxxxxxxxxxxxx>;
devicetree@xxxxxxxxxxxxxxx <devicetree@xxxxxxxxxxxxxxx>;
linux-kernel@xxxxxxxxxxxxxxx <linux-kernel@xxxxxxxxxxxxxxx>;
linux-doc@xxxxxxxxxxxxxxx <linux-doc@xxxxxxxxxxxxxxx>
*Betreff:* Re: [PATCH 1/3] hwmon: max31827: Add PEC support

[External]

On 12/18/23 06:55, Matyas, Daniel wrote:
[ ... ]
On top of that, it is not clear why regmap can't be used in the first
place.
It seems that the major change is that one needs to read the
configuration register after a write to see if there was a PEC
error. It is not immediately obvious why that additional read (if
indeed necessary) would require regmap support to be dropped.

I tried out writing and and reading with regmap, but it is not working
properly. Even if I modify the client flag, I still receive only 2 bytes of data
(a word). I should be receiving 2+1 bytes = data + CRC-8.

With i2c_smbus reads and writes, when I set the flag, I receive the 2+1
bytes, as expected.

The SMBus code in drivers/i2c/i2c-core-smbus.c is supposed to check if
the received PEC is correct for SMBus transfers. Are you saying that
this doesn't work, or that regmap doesn't use SMBus functions to
communicate with the chip ?

Thanks,
Guenter

I am 70% sure, that the regmap does not use SMBus functions.

It should.

$ git grep smbus drivers/base/regmap/regmap-i2c.c
drivers/base/regmap/regmap-i2c.c:static int
regmap_smbus_byte_reg_read(void *context, unsigned int reg,
drivers/base/regmap/regmap-i2c.c:       ret =
i2c_smbus_read_byte_data(i2c, reg);
drivers/base/regmap/regmap-i2c.c:static int
regmap_smbus_byte_reg_write(void *context, unsigned int reg,
drivers/base/regmap/regmap-i2c.c:       return
i2c_smbus_write_byte_data(i2c, reg, val);
drivers/base/regmap/regmap-i2c.c:static const struct regmap_bus
regmap_smbus_byte = {
drivers/base/regmap/regmap-i2c.c:       .reg_write =
regmap_smbus_byte_reg_write,
drivers/base/regmap/regmap-i2c.c:       .reg_read =
regmap_smbus_byte_reg_read,
drivers/base/regmap/regmap-i2c.c:static int
regmap_smbus_word_reg_read(void *context, unsigned int reg,
drivers/base/regmap/regmap-i2c.c:       ret =
i2c_smbus_read_word_data(i2c, reg);
drivers/base/regmap/regmap-i2c.c:static int
regmap_smbus_word_reg_write(void *context, unsigned int reg,
drivers/base/regmap/regmap-i2c.c:       return
i2c_smbus_write_word_data(i2c, reg, val);
drivers/base/regmap/regmap-i2c.c:static const struct regmap_bus
regmap_smbus_word = {
drivers/base/regmap/regmap-i2c.c:       .reg_write =
regmap_smbus_word_reg_write,
drivers/base/regmap/regmap-i2c.c:       .reg_read =
regmap_smbus_word_reg_read,
drivers/base/regmap/regmap-i2c.c:static int
regmap_smbus_word_read_swapped(void *context, unsigned int reg,
drivers/base/regmap/regmap-i2c.c:       ret =
i2c_smbus_read_word_swapped(i2c, reg);
drivers/base/regmap/regmap-i2c.c:static int
regmap_smbus_word_write_swapped(void *context, unsigned int reg,
drivers/base/regmap/regmap-i2c.c:       return
i2c_smbus_write_word_swapped(i2c, reg, val);
drivers/base/regmap/regmap-i2c.c:static const struct regmap_bus
regmap_smbus_word_swapped = {
drivers/base/regmap/regmap-i2c.c:       .reg_write =
regmap_smbus_word_write_swapped,
drivers/base/regmap/regmap-i2c.c:       .reg_read =
regmap_smbus_word_read_swapped,
drivers/base/regmap/regmap-i2c.c:static int
regmap_i2c_smbus_i2c_write(void *context, const void *data,
drivers/base/regmap/regmap-i2c.c:       return
i2c_smbus_write_i2c_block_data(i2c, ((u8 *)data)[0], count,
drivers/base/regmap/regmap-i2c.c:static int
regmap_i2c_smbus_i2c_read(void *context, const void *reg,
drivers/base/regmap/regmap-i2c.c:       ret =
i2c_smbus_read_i2c_block_data(i2c, ((u8 *)reg)[0], val_size, val);
drivers/base/regmap/regmap-i2c.c:static const struct regmap_bus
regmap_i2c_smbus_i2c_block = {
drivers/base/regmap/regmap-i2c.c:       .write =
regmap_i2c_smbus_i2c_write,
drivers/base/regmap/regmap-i2c.c:       .read =
regmap_i2c_smbus_i2c_read,
drivers/base/regmap/regmap-i2c.c:static int
regmap_i2c_smbus_i2c_write_reg16(void *context, const void *data,
drivers/base/regmap/regmap-i2c.c:       return
i2c_smbus_write_i2c_block_data(i2c, ((u8 *)data)[0], count,
drivers/base/regmap/regmap-i2c.c:static int
regmap_i2c_smbus_i2c_read_reg16(void *context, const void *reg,
drivers/base/regmap/regmap-i2c.c:       ret =
i2c_smbus_write_byte_data(i2c, ((u16 *)reg)[0] & 0xff,
drivers/base/regmap/regmap-i2c.c:               ret =
i2c_smbus_read_byte(i2c);
drivers/base/regmap/regmap-i2c.c:static const struct regmap_bus
regmap_i2c_smbus_i2c_block_reg16 = {
drivers/base/regmap/regmap-i2c.c:       .write =
regmap_i2c_smbus_i2c_write_reg16,
drivers/base/regmap/regmap-i2c.c:       .read =
regmap_i2c_smbus_i2c_read_reg16,
drivers/base/regmap/regmap-i2c.c:               bus =
&regmap_i2c_smbus_i2c_block;
drivers/base/regmap/regmap-i2c.c:               bus =
&regmap_i2c_smbus_i2c_block_reg16;
drivers/base/regmap/regmap-i2c.c:                       bus =
&regmap_smbus_word;
drivers/base/regmap/regmap-i2c.c:                       bus =
&regmap_smbus_word_swapped;
drivers/base/regmap/regmap-i2c.c:               bus = &regmap_smbus_byte;

If that doesn't work for some reason, I'd rather figure out why instead of
starting to drop regmap support.

Guenter

I tried to figure it out and this is what I came up with. The code snippet below is from drivers/base/regmap/regmap-i2c.c:

static const struct regmap_bus *regmap_get_i2c_bus(struct i2c_client *i2c,
					const struct regmap_config *config)
{
	const struct i2c_adapter_quirks *quirks;
	const struct regmap_bus *bus = NULL;
	struct regmap_bus *ret_bus;
	u16 max_read = 0, max_write = 0;

	if (i2c_check_functionality(i2c->adapter, I2C_FUNC_I2C))
		bus = &regmap_i2c;
	else if (config->val_bits == 8 && config->reg_bits == 8 &&
		 i2c_check_functionality(i2c->adapter,
					 I2C_FUNC_SMBUS_I2C_BLOCK))
		bus = &regmap_i2c_smbus_i2c_block;
	else if (config->val_bits == 8 && config->reg_bits == 16 &&
		i2c_check_functionality(i2c->adapter,
					I2C_FUNC_SMBUS_I2C_BLOCK))
		bus = &regmap_i2c_smbus_i2c_block_reg16;
	else if (config->val_bits == 16 && config->reg_bits == 8 &&
		 i2c_check_functionality(i2c->adapter,
					 I2C_FUNC_SMBUS_WORD_DATA))
		switch (regmap_get_val_endian(&i2c->dev, NULL, config)) {
		case REGMAP_ENDIAN_LITTLE:
			bus = &regmap_smbus_word;
			break;
		case REGMAP_ENDIAN_BIG:
			bus = &regmap_smbus_word_swapped;
			break;
		default:		/* everything else is not supported */
			break;
		}

This is executed when regmap is initialized. My adapter has the I2C_FUNC_I2C functionality (I use a raspberry pi 4), so it seems to me like regmap_i2c is loaded as the bus. This uses i2c_transfer internally to read and write.

For PEC I need regmap_smbus_word. This uses i2c_smbus_xfer internally. Unlike i2c_transfer, i2c_smbus_xfer can be used to send and receive PEC byte.

What should I do?

It seems to me that regmap should not use functions not supporting PEC if
I2C_CLIENT_PEC is enabled on an i2c device. Of course, that is tricky if
not impossible to implement because the flag can be set at runtime but
the bus function assignment in regmap is static.

The only alternative I can think of is to define driver specific regmap
access functions to re-implement the regmap_smbus_word access functions.
That is less than perfect but better than to drop regmap access entirely.

Copying Mark Brown for additional input.

Thanks,
Guenter