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SubjectRe: [EXT] Re: [PATCH] i2c: pca954x: Add property to skip disabling PCA954x MUX device
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On 2019-09-30 04:43, Biwen Li wrote:
>>
>> On 2019-09-29 12:36, Biwen Li wrote:
>>> On some Layerscape boards like LS2085ARDB and LS2088ARDB, input
>>> pull-up resistors on PCA954x MUX device are missing on board, So, if
>>> MUX are disabled after powered-on, input lines will float leading to
>>> incorrect functionality.

SDA and SCL are not "inputs". They are part of a bus and are both
bidirectional signals. Speaking of input signals in an I2C context
is ambiguous.

>>
>> Hi!
>>
>> Are you saying that the parent bus of the mux is relying on some pull-ups inside
>> the mux?
> Yes, as follows:
> VCC
> -------
> |-------------------
> | |
> \ \
> /10K resister / 10k resister
> \ \
> | |
> | |
> I2C1_SCL ------------------------ I2C1_SCL | ----------------------
> --------------------|SCL | ----------------------------------------|SCL |
> I2C1_SDA | PCA9547 | I2C1_SDA | | PCA9547 |
> --------------------|SDA | ----------------------------|-----------|SDA |
> ------------------------ ----------------------
> --wrong design(need software fix as above or hardware fix)-- --proper design--

Ok, got it (but the "picture" didn't help).

>>
>>> Hence, PCA954x MUX device should never be turned-off after power-on.
>>>
>>> Add property to skip disabling PCA954x MUX device if device tree
>>> contains "i2c-mux-never-disable"
>>> for PCA954x device node.
>>>
>>> Errata ID: E-00013 on board LS2085ARDB and LS2088ARDB (Board revision
>>> found on Rev.B, Rev.C and Rev.D)
>>
>> I think you should follow the example of the i2c-mux-gpio driver and implement
>> the idle-state property instead.
>>
>> That is a lot more consistent, assuming it solves the problem at hand?
> Got it, thanks, I will try it.

I'll wait for that patch then, instead of spending more energy on the
never-disable approach.

Cheers,
Peter
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