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

[Tirdea, Irina]

> -----Original Message-----
> From: Jonathan Cameron [mailto:jic23@xxxxxxxxxx]
> Sent: 05 July, 2015 14:59
> To: Tirdea, Irina; Wolfram Sang; linux-iio@xxxxxxxxxxxxxxx; linux-i2c@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx; 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@xxxxxxxxx>
> 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."

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 */
> >
> >  /**
> >

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