Re: [PATCH v3 1/8] i2c: core: Add support for best effort block read emulation

[Jonathan Cameron <jic23@kernel.org>]

On 10/07/15 18:14, Tirdea, Irina wrote:
> 
> 
>> -----Original Message-----
>> From: Jonathan Cameron [mailto:jic23@kernel.org]
>> Sent: 05 July, 2015 14:59
>> To: Tirdea, Irina; Wolfram Sang; linux-iio@vger.kernel.org; linux-i2c@vger.kernel.org
>> Cc: linux-kernel@vger.kernel.org; Pandruvada, Srinivas; Peter Meerwald
>> Subject: Re: [PATCH v3 1/8] i2c: core: Add support for best effort block read emulation
>>
>> On 03/07/15 10:33, 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@intel.com>
>> Looks good to me - I vaguely wondered if it would make sense to use
>> an endian conversion in the word case, but as we have possible odd
>> numbers of bytes that gets fiddly.
>>
> 
> Thanks for the review, Jonathan!
> 
>> I wonder what devices do if you do a word read beyond their end address?
>> Perhaps in odd cases we should always fall back to byte reads?
> 
> In my tests I can read beyond the end address, but I cannot be sure if this is OK for all
> devices. This was actually a suggestion from Wolfram for v1, but maybe I'm missing
> something.
> 
> Wolfram, is it safe to read one byte beyond the end address or should I better use
> only byte reads for odd lengths?
> 
>>
>>> ---
>>>  drivers/i2c/i2c-core.c | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++
>>>  include/linux/i2c.h    |  3 +++
>>>  2 files changed, 63 insertions(+)
>>>
>>> diff --git a/drivers/i2c/i2c-core.c b/drivers/i2c/i2c-core.c
>>> index 96771ea..55a3455 100644
>>> --- a/drivers/i2c/i2c-core.c
>>> +++ b/drivers/i2c/i2c-core.c
>>> @@ -2914,6 +2914,66 @@ trace:
>>>  }
>>>  EXPORT_SYMBOL(i2c_smbus_xfer);
>>>
>>> +/**
>>> + * 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
>>> + * @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.
>>> + *
>>> + * Before using this function you must double-check if the I2C slave does
>>> + * support exchanging a block transfer with a byte transfer.
>> Add something here about addressing assumptions.  You get odd devices which
>> will give bulk reads of addresses not mapped to a nice linear region when
>> you do byte reads.
> 
> OK, I'll add this to the comment above:
> "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."
> 
Works for me.
> Thanks,
> Irina
> 
>>> + */
>>> +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);
>>> +	} else if (i2c_check_functionality(client->adapter,
>>> +					   I2C_FUNC_SMBUS_READ_WORD_DATA)) {
>>> +		for (i = 0; i < length; i += 2) {
>>> +			status = i2c_smbus_read_word_data(client, command + i);
>>> +			if (status < 0)
>>> +				return status;
>>> +			values[i] = status & 0xff;
>>> +			if ((i + 1) < length)
>>> +				values[i + 1] = status >> 8;
>>> +		}
>>> +		if (i > length)
>>> +			return length;
>>> +		return i;
>>> +	} else 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;
>>> +	}
>>> +
>>> +	dev_err(&client->adapter->dev, "Unsupported transactions: %d,%d,%d\n",
>>> +		I2C_SMBUS_I2C_BLOCK_DATA, I2C_SMBUS_WORD_DATA,
>>> +		I2C_SMBUS_BYTE_DATA);
>>> +
>>> +	return -EOPNOTSUPP;
>>> +}
>>> +EXPORT_SYMBOL(i2c_smbus_read_i2c_block_data_or_emulated);
>>> +
>>>  #if IS_ENABLED(CONFIG_I2C_SLAVE)
>>>  int i2c_slave_register(struct i2c_client *client, i2c_slave_cb_t slave_cb)
>>>  {
>>> diff --git a/include/linux/i2c.h b/include/linux/i2c.h
>>> index e83a738..faf518d 100644
>>> --- a/include/linux/i2c.h
>>> +++ b/include/linux/i2c.h
>>> @@ -121,6 +121,9 @@ extern s32 i2c_smbus_read_i2c_block_data(const struct i2c_client *client,
>>>  extern s32 i2c_smbus_write_i2c_block_data(const struct i2c_client *client,
>>>  					  u8 command, u8 length,
>>>  					  const u8 *values);
>>> +extern s32
>>> +i2c_smbus_read_i2c_block_data_or_emulated(const struct i2c_client *client,
>>> +					  u8 command, u8 length, u8 *values);
>>>  #endif /* I2C */
>>>
>>>  /**
>>>
>