linux-kernel - Re: [PATCH v4 1/8] i2c: core: Add support for best effort block read emulation

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Date:	Sun, 9 Aug 2015 09:31:36 +0200
From:	Wolfram Sang <wsa@...-dreams.de>
To:	Irina Tirdea <irina.tirdea@...el.com>
Cc:	Jonathan Cameron <jic23@...nel.org>, linux-iio@...r.kernel.org,
	linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
	Srinivas Pandruvada <srinivas.pandruvada@...el.com>,
	Peter Meerwald <pmeerw@...erw.net>
Subject: Re: [PATCH v4 1/8] i2c: core: Add support for best effort block read
 emulation

On Tue, Aug 04, 2015 at 05:04:09PM +0300, Irina Tirdea wrote:
> There are devices that need to handle block transactions
> regardless of the capabilities exported by the adapter.
> For performance reasons, they need to use i2c read blocks
> if available, otherwise emulate the block transaction with word
> or byte transactions.
> 
> Add support for a helper function that would read a data block
> using the best transfer available: I2C_FUNC_SMBUS_READ_I2C_BLOCK,
> I2C_FUNC_SMBUS_READ_WORD_DATA or I2C_FUNC_SMBUS_READ_BYTE_DATA.
> 
> Signed-off-by: Irina Tirdea <irina.tirdea@...el.com>

We are close, but I think there is one optimization left to do.

> +/**
> + * i2c_smbus_read_i2c_block_data_or_emulated - read block or emulate
> + * @client: Handle to slave device
> + * @command: Byte interpreted by slave
> + * @length: Size of data block; SMBus allows at most 32 bytes

Please refer to I2C_SMBUS_BLOCK_MAX instead of 32. The new SMBus specs
increased this amount to 256. (Yes, we don't support that yet.)

> + * @values: Byte array into which data will be read; big enough to hold
> + *	the data returned by the slave.  SMBus allows at most 32 bytes.
> + *
> + * This executes the SMBus "block read" protocol if supported by the adapter.
> + * If block read is not supported, it emulates it using either word or byte
> + * read protocols depending on availability.
> + *
> + * The addresses of the I2C slave device that are accessed with this function
> + * must be mapped to a linear region, so that a block read will have the same
> + * effect as a byte read. Before using this function you must double-check
> + * if the I2C slave does support exchanging a block transfer with a byte
> + * transfer.
> + */
> +s32 i2c_smbus_read_i2c_block_data_or_emulated(const struct i2c_client *client,
> +					      u8 command, u8 length, u8 *values)
> +{
> +	u8 i;
> +	int status;
> +
> +	if (length > I2C_SMBUS_BLOCK_MAX)
> +		length = I2C_SMBUS_BLOCK_MAX;
> +
> +	if (i2c_check_functionality(client->adapter, I2C_FUNC_SMBUS_READ_I2C_BLOCK))
> +		return i2c_smbus_read_i2c_block_data(client, command, length, values);
> +
> +	if (i2c_check_functionality(client->adapter, I2C_FUNC_SMBUS_READ_WORD_DATA |
> +				    I2C_FUNC_SMBUS_READ_BYTE_DATA)) {
> +		for (i = 0; (i + 2) <= length; i += 2) {
> +			status = i2c_smbus_read_word_data(client, command + i);
> +			if (status < 0)
> +				return status;
> +			values[i] = status & 0xff;
> +			values[i + 1] = status >> 8;
> +		}
> +		if (i < length) {
> +			status = i2c_smbus_read_byte_data(client, command + i);
> +			if (status < 0)
> +				return status;
> +			values[i] = status;
> +			i++;
> +		}
> +		return i;
> +	}
> +
> +	if (i2c_check_functionality(client->adapter, I2C_FUNC_SMBUS_READ_BYTE_DATA)) {
> +		for (i = 0; i < length; i++) {
> +			status = i2c_smbus_read_byte_data(client, command + i);
> +			if (status < 0)
> +				return status;
> +			values[i] = status;
> +		}
> +		return i;
> +	}

We have two very similar blocks for transferring READ_BYTE_DATA now.
What about this pseudo code:

	if (check_func(I2C_BLOCK))
		return i2c_smbus_read_i2c_block_data(...);

	if (!check_func(I2C_READ_BYTE_DATA)
		return -EOPNOTSUPP;

	if (check_func(I2C_READ_WORD_DATA)
		read_as_many_words_as_possible;

	read_all_the_remaining_bytes;


> +	dev_err(&client->adapter->dev, "Unsupported transactions\n");

Again, if you want to keep this one, it should be the client device
reporting the error.

Thanks,

   Wolfram


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