Re: [PATCH i2c-tools] i2ctransfer: add support for I2C_M_RECV_LEN

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> On Aug 2, 2020, at 2:38 PM, Wolfram Sang <wsa+renesas@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> Hi Daniel,
> 
>>> +		__u16 len = recv_len ? msgs[i].buf[0] + 1 : msgs[i].len;
>> 
>> This is post-ioctl()? (Sorry, still not familiar enough with i2c-tools.)
> 
> Yes, read buffers are only printed after the ioctl. And 'print_msgs' is
> probably the most complex function within this tool :/
> 
>> It isn’t wrong, but shouldn’t be necessary.
>> Unless the adapter driver you’re using went astray. Not ruling that out.
> 
> I have just checked existing I2C_M_RECV_LEN handling. Quite some drivers
> do it wrong. And there is no consistency in what they return. Lots of
> things to fix there...

Would be curious about what variants are there.

Note that msgs[i].len isn’t updated, you only get <extra_bytes> of data back,
so the difference would be severe: msgs[i].len is what guides copy_to_user().

https://github.com/torvalds/linux/blob/master/drivers/i2c/i2c-dev.c#L301

Daniel





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