Re: Problem with a hih6130 sensor in a OMAP I2C bus

["José Miguel Gonçalves" <jose.goncalves-ojS98SfuVkU@public.gmane.org>]

Hi Jean,

On 09-12-2013 12:22, Jean Delvare wrote:
> Hi José,
>
> On Mon, 09 Dec 2013 11:42:13 +0000, José Miguel Gonçalves wrote:
>> While testing an HIH6130 humidity and temperature sensor with I2C
>> interface in a BeagleBone board I've found that I was unable to read it
>> because the driver always returned EINVAL. With some debugging I've
>> found that the error was due to a test on omap_i2c_xfer_msg() in OMAP
>> I2C driver that invalidates zero length writes. The hwmon hih6130 driver
>> issues such kind of request in hih6130_update_measurements() to issue a
>> measurement request to the sensor.
>>
>> I was able to get measurements from the sensor by hacking the hih6130
>> driver replacing the following line in hih6130_update_measurements();
>>
>>           ret = i2c_master_send(client, tmp, 0);
>>
>> by
>>
>>           tmp[0] = 0;
>>           ret = i2c_master_send(client, tmp, 1);
>>
>> Is this the correct way to fix this issue, or should the fix be in the
>> I2C OMAP driver to accept zero length transfers?
> Ideally it should be fixed in the i2c-omap bus driver. However, if
> zero-length transfers are explicitly prohibited, it may mean that the
> hardware itself can't do it (in which case adding a comment saying so
> in the source code would be great.) You'll have to check. The driver
> supports several generations of OMAP chipsets so you'll have to check
> them all, maybe some can handle zero-length messages and some do not.
>
> If fixing it in the i2c-omap driver is not possible, then working
> around it in the hih6130 driver would be acceptable. You'll have to
> make sure there is no side effect to sending a single byte to the chip.
> If performance is a concern, you may have to enable the workaround only
> when zero-byte messages aren't supported. You can check with:
>
> i2c_get_functionality(i2c_adapter) & I2C_FUNC_SMBUS_QUICK
>

I am by no means an expert on OMAP platforms and/or I2C drivers so, if 
anyone else with more knowledge of them has a better suggestion, I will 
follow your suggestion and submit my workaround patch for the hih6130 
driver with the added check for support of zero-byte messages by the 
underlying I2C driver. From my understanding of the technical note for 
I2C communication on this chip;

http://www.mouser.com/pdfdocs/I2CCommunications.pdf

I see no problems on writing that dummy byte to it and my tests on a 
real hardware also show that.

Best regards,
José Gonçalves