RE: [RFC PATCH 1/1] i2c: core: Add support for best effort block read emulation

["Tirdea, Irina" <irina.tirdea@intel.com>]

> -----Original Message-----
> From: Wolfram Sang [mailto:wsa@the-dreams.de]
> Sent: 15 April, 2015 18:55
> To: Tirdea, Irina
> Cc: linux-i2c@vger.kernel.org; linux-kernel@vger.kernel.org
> Subject: Re: [RFC PATCH 1/1] i2c: core: Add support for best effort block read emulation
> 
> On Wed, Mar 25, 2015 at 04:33:20PM +0200, 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>
> > ---
> >
> > Hi,
> >
> > This is a new API proposal to handle i2c block emulation in the
> > core instead of the driver code.
> >
> > This is needed for a set of iio sensor changes ([1], [2], [3])
> > that would otherwise duplicate this code. There are also some
> > usages of this functionality in the kernel (e.g. eeprom driver at24).
> >
> > Please let me know what you think.
> 
> I am open to add something like this. One change I'd like to request is
> to introduce a user, e.g. convert at24.
> 
Thanks for the review!

Sure, I'll send a new version with the fixes you suggested and include a user as well.
> >
> > Thanks,
> > Irina
> >
> > [1] https://lkml.org/lkml/2015/2/16/408
> > [2] https://lkml.org/lkml/2015/2/16/413
> > [3] https://lkml.org/lkml/2015/2/16/402
> >
> >  drivers/i2c/i2c-core.c | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++
> >  include/linux/i2c.h    |  3 +++
> >  2 files changed, 65 insertions(+)
> >
> > diff --git a/drivers/i2c/i2c-core.c b/drivers/i2c/i2c-core.c
> > index fe80f85..2579f7d 100644
> > --- a/drivers/i2c/i2c-core.c
> > +++ b/drivers/i2c/i2c-core.c
> > @@ -2907,6 +2907,68 @@ 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.
> 
> Sidenote: SMBus3 allows 255 byte, but we don't support that (yet), so
> this is okay for now.
> 
> > + *
> > + * 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.
> 
> Here I'd like to see a warning that people should double-check if their
> I2C slave does support that. Sometimes one can't exchange a block
> transfer with a byte transfer.
> 
Ok.
> > + */
> > +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 |
> > +					   I2C_FUNC_SMBUS_READ_BYTE_DATA)) {
> 
> What about skipping the need for READ_BYTE_DATA and dump the byte which
> was maybe read too much?
> 
Yes, that will simply the code.
> > +		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;
> > +	} 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);
> 
> Thanks,
> 
>    Wolfram