Date:   Tue, 27 Nov 2018 18:51:13 +0000
From:   Peter Rosin <peda@...ntia.se>
To:     Luca Ceresoli <luca@...aceresoli.net>,
        Luca Ceresoli <lucaceresoli77@...il.com>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Wolfram Sang <wsa@...-dreams.de>
Subject: Re: [PATCH v3] i2c: mux: remove duplicated i2c_algorithm

On 2018-11-18 12:13, Luca Ceresoli wrote:
> Hi Peter,
> 
> On 10/10/18 17:48, Luca Ceresoli wrote:
>> Hi Peter,
>>
>> On 08/10/2018 23:43, Peter Rosin wrote:
>>> On 2018-10-03 17:19, Luca Ceresoli wrote:
>>>> From: Luca Ceresoli <luca@...aceresoli.net>
>>>>
>>>> i2c-mux instantiates one i2c_algorithm for each downstream adapter.
>>>> However these algorithms are all identical, depending only on the
>>>> parent adapter.
>>>>
>>>> Avoid duplication by hoisting the i2c_algorithm from the adapters to
>>>> the i2c_mux_core object, and reuse it in all the adapters.
>>>
>>> Ouch, while I like the concept of having one i2c_algorithm per mux,
>>> this patch is not working. Various i2c-mux drivers set the
>>> muxc->mux_locked variable *after* the i2c_mux_alloc call, and this
>>> patch breaks such use.
> 
> I finally had a look into this issue. Three drivers are setting
> mux_locked after i2c_mux_alloc: i2c-mux-gpmux, i2c-mux-gpio and
> i2c-mux-pinctrl.
> 
> i2c-mux-gpmux is trivial to fix.
> 
> The other two are not trivial because:
> 
>  1. they compute mux_locked from other variables
>  2. those variables are stored in the drivers "private" data
>  3. their private data is stored inside struct i2c_mux_core
>     (muxc->priv) which exists only after i2c_mux_alloc()
> 
> In those cases computing mux_locked before i2c_mux_alloc() involves
> quite invasive changes. It took 3 non-trivial commits just for
> i2c-mux-gpio, and I still have to look into i2c-mux-pinctrl.
> 
> So the question is: do we really want to do this?
> 
> Using the private storage provided by i2c_mux_alloc() is a handy
> feature, at least for simpler drivers which know in advance the flags
> they need to set. OTOH I don't like individual drivers to manipulate
> mux_core flags that look very much like internal data. It makes any
> change to the i2c mux core harder, since every changed line could have
> side effects in some drivers, which is what's happening here.
> 
> What's your opinion about this issue?

I obviously don't like that drivers are poking around in struct
i2c_mux_core.

But, your description sounds precisely how I remembered it. The
underlying problem is of course that i2c-mux-gpio and
i2c-mux-pinctrl do really nasty digs into internal parts of the
gpio and the pinctrl subsystems as they *try* to figure out if
they should be mux-locked or parent-locked. The result of that
digging is not completely reliable, but it solves the issue
without help from device-tree properties in at least one case
that I know about. However, for that case I also know that there
is no risk of regression since I control the distribution of
both kernel and .dtb for any upgrade. Anyway, it was done like
it was since I at the time did not dare to question the feedback
from the device-tree camp, and actually thought it was a good
thing, and thus did not push for a device-tree property when
Rob complained about the property not describing HW and instead
was just working around kernel issues [1]. The mux-locked vs.
parent-locked property has been added since. In retrospect, the
whole attempt to auto-detect mux-locked or parent-locked was a
mistake, and everything would have been so much easier if the
device-tree could always just state what the requirement is. At
least that's my current thoughts on the matter. Maybe we should
attempt to remove the ugly auto-detect code and see if anyone
complains?

But of course, another aspect is that not everything is DT, so
perhaps there is no clean solution?

Cheers,
Peter

[1] https://lkml.org/lkml/2016/1/6/437

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