Hi Aaron,
On Mon, 9 Nov 2015 18:25:11 -0600 (CST), Aaron Sierra wrote:
> Previously, the at24 driver would bail out in the case of a 16-bit
> addressable EEPROM attached to an SMBus controller. This is because
> SMBus block reads and writes don't map to I2C multi-byte reads and
> writes when the offset portion is 2 bytes.
>
> Instead of bailing out, this patch settles for functioning with single
> byte read SMBus cycles. Writes can be block or single-byte, depending
> on SMBus controller features.
>
> Read access is not without some risk. Multiple SMBus cycles are
> required to read even one byte. If the SMBus has multiple masters and
> one accesses this EEPROM between the dummy address write and the
> subsequent current-address-read cycle(s), this driver will receive
> data from the wrong address.
>
> Functionality has been tested with the following devices:
>
> AT24CM01 attached to Intel ISCH SMBus
> AT24C512 attached to Intel I801 SMBus
>
> Read performance:
> 3.6 KB/s with 32-byte* access
>
> *limited to 32-bytes by I2C_SMBUS_BLOCK_MAX.
>
> Write performance:
> 248 B/s with 1-byte page (default)
> 3.9 KB/s with 128-byte* page (via platform data)
>
> *limited to 31-bytes by I2C_SMBUS_BLOCK_MAX - 1.
>
> Signed-off-by: Nate Case <ncase@xxxxxxxxxxx>
> Signed-off-by: Aaron Sierra <asierra@xxxxxxxxxxx>
> ---
> v2 - Account for changes related to introduction of
> i2c_smbus_read_i2c_block_data_or_emulated()
> v3 - Consolidate three patches into one
> - Expand comments regarding SMBus multi-master read risks.
> - Rely on current-address-read for improved read performance (i.e. one
> dummy address write followed by multiple individual byte reads).
> This improves performance from 1.4 KiB/s to 3.6 KiB/s.
> - Use struct at24_data's writebuf instead of kzalloc-ing
> - Only limit write_max by 1-byte when accessing a 16-bit device with
> block writes instead of attempting to preserve a power-of-two.
> - Style fixes (indentation, parentheses, unnecessary masking, etc.)
> v4 - Address 16-bit safety in Kconfig
> - Set "count" to zero later in at24_smbus_read_block_data()
> - Fix over-80-columns issues in at24_eeprom_read()
> - Fix write_max off-by-one in at24_probe()
> - Check SMBus functionality needed for 16-bit device reads
> - Homogenize indentation of SMBus functionality checks for SMBus write
>
> drivers/misc/eeprom/Kconfig | 5 +-
> drivers/misc/eeprom/at24.c | 129 +++++++++++++++++++++++++++++++++++++++-----
> 2 files changed, 119 insertions(+), 15 deletions(-)
This is a significant addition of code so feel free to add your name at
the top of at24.c.
We're almost there:
> diff --git a/drivers/misc/eeprom/at24.c b/drivers/misc/eeprom/at24.c
> index 5d7c090..3dfd2ed 100644
> --- a/drivers/misc/eeprom/at24.c
> +++ b/drivers/misc/eeprom/at24.c
> (...)
> @@ -527,10 +608,19 @@ static int at24_probe(struct i2c_client *client, const struct i2c_device_id *id)
>
> /* Use I2C operations unless we're stuck with SMBus extensions. */
> if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C)) {
> - if (chip.flags & AT24_FLAG_ADDR16)
> - return -EPFNOSUPPORT;
> -
> - if (i2c_check_functionality(client->adapter,
> + if ((chip.flags & AT24_FLAG_ADDR16) &&
> + i2c_check_functionality(client->adapter,
> + I2C_FUNC_SMBUS_READ_BYTE_DATA |
> + I2C_FUNC_SMBUS_WRITE_BYTE_DATA)) {
It is I2C_FUNC_SMBUS_READ_BYTE that you use, not
I2C_FUNC_SMBUS_READ_BYTE_DATA.
> + /*
> + * We need SMBUS_WRITE_BYTE_DATA and
> + * SMBUS_READ_BYTE_DATA to implement byte reads for
> + * 16-bit address devices. This will be slow, but
> + * better than nothing (e.g. read @ 3.6 KiB/s). It is
> + * also unsafe in a multi-master topology.
> + */
> + use_smbus = I2C_SMBUS_BYTE_DATA;
> + } else if (i2c_check_functionality(client->adapter,
> I2C_FUNC_SMBUS_READ_I2C_BLOCK)) {
> use_smbus = I2C_SMBUS_I2C_BLOCK_DATA;
> } else if (i2c_check_functionality(client->adapter,
> (...)
> @@ -598,8 +698,9 @@ static int at24_probe(struct i2c_client *client, const struct i2c_device_id *id)
>
> if (write_max > io_limit)
> write_max = io_limit;
> - if (use_smbus && write_max > I2C_SMBUS_BLOCK_MAX)
> - write_max = I2C_SMBUS_BLOCK_MAX;
> + if (use_smbus && write_max >= I2C_SMBUS_BLOCK_MAX)
> + write_max = I2C_SMBUS_BLOCK_MAX -
> + !!(chip.flags & AT24_FLAG_ADDR16);
Beh. OK, it works, I will admit it's even kind of clever, but it also
looks fragile and confusing to some degree. What is wrong with just
spelling out the condition explicitly?
unsigned smbus_limit = (chip.flags & AT24_FLAG_ADDR16) ?
I2C_SMBUS_BLOCK_MAX - 1 :
I2C_SMBUS_BLOCK_MAX;
if (use_smbus && write_max > smbus_limit)
write_max = smbus_limit;
This might not even be slower, and IMHO it is easier to understand.
> at24->write_max = write_max;
>
> /* buffer (data + address at the beginning) */
I have no objection to this patch being merged into the upstream
kernel, but ultimately this is Wolfram's call.
Reviewed-by: Jean Delvare <jdelvare@xxxxxxx>
--
Jean Delvare
SUSE L3 Support
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