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Date: Sat, 11 Jul 2015 18:40:35 +0100
From: Jonathan Cameron <jic23@...nel.org>
To: "Tirdea, Irina" <irina.tirdea@...el.com>,
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
On 10/07/15 18:14, Tirdea, Irina wrote:
>
>
>> -----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."
>
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 */
>>>
>>> /**
>>>
>
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