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

Daniel Stodden
2020-08-03T07:30:50Z

> On Aug 2, 2020, at 2:38 PM, Wolfram Sang <wsa+renesas@sang-engineering.com> 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