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Date: Fri, 10 Jul 2015 17:14:27 +0000
From: "Tirdea, Irina" <irina.tirdea@...el.com>
To: Jonathan Cameron <jic23@...nel.org>,
Wolfram Sang <wsa@...-dreams.de>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"Pandruvada, Srinivas" <srinivas.pandruvada@...el.com>,
Peter Meerwald <pmeerw@...erw.net>,
"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>
Subject: RE: [PATCH v3 1/8] i2c: core: Add support for best effort block
read emulation
> -----Original Message-----
> From: Jonathan Cameron [mailto:jic23@...nel.org]
> Sent: 05 July, 2015 14:59
> To: Tirdea, Irina; Wolfram Sang; linux-iio@...r.kernel.org; linux-i2c@...r.kernel.org
> Cc: linux-kernel@...r.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@...el.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."
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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