[U-Boot] Regression: dm: i2c: Make i2c_get_chip_for_busnum() fail if the chip is not detected

[Stephen Warren]

On 12/11/18 2:44 AM, Jean-Jacques Hiblot wrote:
> 
> On 11/12/2018 05:41, Heiko Schocher wrote:
>> Hello Stephen,
>>
>> Am 10.12.2018 um 19:23 schrieb Stephen Warren:
>>> The following commit:
>>>
>>>> dm: i2c: Make i2c_get_chip_for_busnum() fail if the chip is not 
>>>> detected
>>>>     i2c_get_chip_for_busnum() really should check the presence of 
>>>> the chip on
>>>>     the bus. Most of the users of this function assume that this is 
>>>> done.
>>>
>>> ... causes a boot failure on NVIDIA Jetson TX2:
>>
>> :-(
>>
>> Thanks for detecting so fast!
>>
>>>> U-Boot 2019.01-rc1-00220-g7ff485c68b7e (Dec 10 2018 - 11:20:41 -0700)
>>>>
>>>> TEGRA186
>>>> Model: NVIDIA P2771-0000-500
>>>> DRAM:  7.8 GiB
>>>> tegra_ivc_read_get_next_frame() timed out (-12)
>>>> tegra_board_init: Cannot find MAX77620 I2C chip
>>>> initcall sequence 00000000fffa95a8 failed at call 0000000080083480 
>>>> (err=-110)
>>>> ### ERROR ### Please RESET the board ###
>>>
>>> This may be due to the fact the bus in question is implemented by RPC 
>>> to a separate CPU, and that mechanism hasn't been used with probing 
>>> before. In general though, there's not guarantee that probing will 
>>> work even on a local/native I2C bus, since different chips don't 
>>> support all probe methods (see i2c-detect in Linux, which supports 
>>> various different probing methods due to this), so I'm rather 
>>> surprised this change was implemented. Is it really necessary? I 
>>> believe we should revert it.
> 
> The probe method is not the same in u-boot as in i2c-detect. In u-boot 
> there is no data transfer, the probe only sends the address on the bus 
> and fails if the device does not respond with a ACK (or if something 
> else goes wrong). Every I2C device supports this kind of probe by design.
> 
> Errors could happen though:
> 
> - device not present, or not powered up or in reset state
> 
> - bus not ready (in your case, maybe the CPU doing the actual work is 
> not ready)
> 
> - bus speed too high.
> 
> In all those cases this could be fixed in the board specific code.
> 
> 
> While I agree that a commit should not break platforms, I'm not 
> convinced that reverting the commit is the right solution: in 
> tegra_board_init() the call to i2c_get_chip_for_busnum() is followed by 
> a call to dm_i2c_write(). Assuming that we remove the offending commit, 
> i2c_get_chip_for_busnum() would not fail anymore, but in this case the 
> following call to dm_i2c_write() should fail. If it doesn't then I 
> suspect that there is something wrong in the tegra I2C bus driver that 
> makes it unable to transfer only the address word.

Yes, I imagine that our other CPU doesn't support zero-length transfers. 
However, that's not going to change. Our only choice is not to do this 
unnecessary probing.

Even if we were going to modify the Tegra I2C bus driver to solve or 
work around this, we would still need to:

a) Revert the change.
b) Develop the fix.
c) Re-apply the original change.

... to reduce the time window where the code is broken. Right now 
everyone working on Tegra U-Boot is rather swamped so spending time on 
fixing this regression is rather annoying...