linux-kernel - Re: [PATCH v2 1/4] i2c/busses: Add i2c-icy for I2C on m68k/Amiga

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Date:   Thu, 15 Aug 2019 13:52:47 +0200
From:   Max <max@...as.org>
To:     Wolfram Sang <wsa@...-dreams.de>
Cc:     linux-i2c@...r.kernel.org, linux-hwmon@...r.kernel.org,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>, linux-m68k@...r.kernel.org,
        linux-kernel@...r.kernel.org, glaubitz@...sik.fu-berlin.de
Subject: Re: [PATCH v2 1/4] i2c/busses: Add i2c-icy for I2C on m68k/Amiga

On 08/15/2019 01:48 PM, Wolfram Sang wrote:
>> Well, the other option is to remove it, and then add it back once
>> somebody complains - which is unlikely to happen. The clock parameter
>> is PCF8584 specific anyway, and  I think removing it is a good option,
> 
> My suggestion is to do that incrementally. First, get your driver
> accepted. Second, do the cleanups which affect elektor as well later.

My suggestion is to not touch i2c-elektor at all for now, and remove the 'clock' parameter from the first iteration of i2c-icy. It can be added back in case someone complains, which I deem unlikely.

>> as I've done the same with getown() (where in i2c-elektor, 'own' sets
>> the PCF8584's own address).
> 
> I wondered about that. Can the PCF8584 really act as a slave, too?
> Somewhen I need to check its datasheet.

Yes it can, but this driver does not implement this.

In fact, I just remembered that the own address, while set, is not used at all in master mode!

Quote from the datasheet:
https://www.nxp.com/docs/en/data-sheet/PCF8584.pdf

  6.4 Own address register S0'

  When the PCF8584 is addressed as slave, this register
  must be loaded with the 7-bit I 2 C-bus address to which the
  PCF8584 is to respond. During initialization, the own
  address register S0' must be written to, regardless
  whether it is later used.

>> Question is, if I remove the parameter, I'd like it to be
>> non-destructive. Do you know of anything that can go wrong if the I2C
>> master is running the bus on a wrong clock?
> 
> Not sure if I understand you correctly, but if the bus freq is too fast
> then devices won't respond. Too slow is not a problem.

Okay, so we don't care. Cool, then it's safe to kick the 'clock' parameter from i2c-icy. All 2019 boards (which should be the vast majority in existence) came with a 12 MHz oscillator AFAIK, so the default should be good.

Max