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Subject: Re: [PATCH v4 01/10] i3c: Add core I3C infrastructure
To:     Boris Brezillon <boris.brezillon@bootlin.com>,
        Arnd Bergmann <arnd@arndb.de>
Cc:     Wolfram Sang <wsa@the-dreams.de>, linux-i2c@vger.kernel.org,
        Jonathan Corbet <corbet@lwn.net>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Przemyslaw Sroka <psroka@cadence.com>,
        Arkadiusz Golec <agolec@cadence.com>,
        Alan Douglas <adouglas@cadence.com>,
        Bartosz Folta <bfolta@cadence.com>,
        Damian Kos <dkos@cadence.com>,
        Alicja Jurasik-Urbaniak <alicja@cadence.com>,
        Cyprian Wronka <cwronka@cadence.com>,
        Suresh Punnoose <sureshp@cadence.com>,
        Rafal Ciepiela <rafalc@cadence.com>,
        Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
        Nishanth Menon <nm@ti.com>, Rob Herring <robh+dt@kernel.org>,
        Pawel Moll <pawel.moll@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Ian Campbell <ijc+devicetree@hellion.org.uk>,
        Kumar Gala <galak@codeaurora.org>,
        DTML <devicetree@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Vitor Soares <Vitor.Soares@synopsys.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Xiang Lin <Xiang.Lin@synaptics.com>, linux-gpio@vger.kernel.org
References: <20180330074751.25987-1-boris.brezillon@bootlin.com>
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 <20180711191212.3855bb25@bbrezillon>
From:   Peter Rosin <peda@axentia.se>
Organization: Axentia Technologies AB
Message-ID: <a31dc625-56bf-7a59-d9a3-2841360b864e@axentia.se>
Date:   Thu, 12 Jul 2018 06:41:15 +0200
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[tried to send something like this yesterday, but it appears to have been
lost, sorry for any duplicate]

On 2018-07-11 19:12, Boris Brezillon wrote:
> On Wed, 11 Jul 2018 17:39:56 +0200
> Arnd Bergmann <arnd@arndb.de> wrote:
> 
>> On Wed, Jul 11, 2018 at 4:41 PM, Boris Brezillon
>> <boris.brezillon@bootlin.com> wrote:
>>> On Wed, 11 Jul 2018 16:01:56 +0200 Arnd Bergmann <arnd@arndb.de> wrote:  
>>>>> - the bus element is a separate object and is not implicitly described
>>>>>   by the master (as done in I2C). The reason is that I want to be able
>>>>>   to handle multiple master connected to the same bus and visible to
>>>>>   Linux.
>>>>>   In this situation, we should only have one instance of the device and
>>>>>   not one per master, and sharing the bus object would be part of the
>>>>>   solution to gracefully handle this case.
>>>>>   I'm not sure we will ever need to deal with multiple masters
>>>>>   controlling the same bus and exposed under Linux, but separating the
>>>>>   bus and master concept is pretty easy, hence the decision to do it
>>>>>   like that.
>>>>>   The other benefit of separating the bus and master concepts is that
>>>>>   master devices appear under the bus directory in sysfs.  
>>>>
>>>> I'm not following here at all, sorry for missing prior discussion if this
>>>> was already explained. What is the "multiple master" case? Do you
>>>> mean multiple devices that are controlled by Linux and that each talk
>>>> to other devices on the same bus, multiple operating systems that
>>>> have talk to are able to own the bus with the kernel being one of
>>>> them, a controller that controls multiple independent buses,
>>>> or something else?  
>>>
>>> I mean several masters connected to the same bus and all exposed to the
>>> same Linux instance. In this case, the question is, should we have X
>>> I3C buses exposed (X being the number of masters) or should we only
>>> have one?
>>>
>>> Having a bus represented as a separate object allows us to switch to
>>> the "one bus : X masters" representation if we need too.  
>> ...
>>>>
>>>> This feels a bit odd: so you have bus_type that can contain devices
>>>> of three (?) different device types: i3c_device_type, i3c_master_type
>>>> and i3c_busdev_type.
>>>>
>>>> Generally speaking, we don't have a lot of subsystems that even
>>>> use device_types. I assume that the i3c_device_type for a
>>>> device that corresponds to an endpoint on the bus, but I'm
>>>> still confused about the other two, and why they are part of
>>>> the same bus_type.  
>>>
>>> i3c_busdev is just a virtual device representing the bus itself.
>>> i3c_master is representing the I3C master driving the bus. The reason
>>> for having a different type here is to avoid attaching this device to a
>>> driver but still being able to see the master controller as a device on
>>> the bus. And finally, i3c_device are all remote devices that can be
>>> accessed through a given i3c_master.
>>>
>>> This all comes from the design choice I made to represent the bus as a
>>> separate object in order to be able to share it between different
>>> master controllers exposed through the same Linux instance. Since
>>> master controllers are also remote devices for other controllers, we
>>> need to represent them.  
>>
>> Ok, so I think this is the most important question to resolve: do we
>> actually need to control multiple masters on a single bus from one OS
>> or not?
>>
>> The problem that I see is that it breaks the tree abstraction that
>> we use in the dtb interface, in the driver model and in sysfs.
>> If we need to deal with a hardware bus structure like
>>
>>               cpu
>>              /   \
>>             /     \
>>        platdev   platdev
>>            |        |
>>      i3c-master   i3c-master
>>             \      /
>>              \    /
>>             i3c-bus
>>              /    \
>>          device   device
>>
>> then that abstraction no longer holds. Clearly you could build
>> a system like that, and if we have to support it, the i3c infrastructure
>> should be prepared for it, since we wouldn't be able to retrofit
>> it later.
> 
> Exactly. For the DT representation I thought we could have the primary
> master hold the device nodes, and then have secondary masters reference
> the main master with a phandle (i3c-bus = <&main_i3c_master>;). For the
> sysfs representation, it would be the same. Only one of the master
> would create the i3c_bus object and the other masters would just
> reference it.
> 
>>
>> What would be the point of building such a system though?
> 
> This, I don't know. But as you said, if we go for a "one bus per
> master" representation, going back will be difficult.
> 
>> Is this for performance, failover, or something else?
> 
> No, I don't think so, especially since the mastership handover
> operation is not free. So keeping the same master in control is
> probably better in term of perfs.
> 
> One case I can think of is when the primary master does not have enough
> resources to address all devices on the bus, and let the secondary
> master handle all transactions targeting those devices.
> 
>> IOW, what feature would we lose if we were to declare that
>> setup above invalid (and ensure you cannot represent it in DT)?
> 
> That's exactly the sort of discussion I wanted to trigger. Maybe we
> shouldn't care and expose this use case as if it was X different I3C
> buses (with all devices present on the bus being exposed X times to the
> system).

For I2C, this multiple masters for one bus case was retrofitted in
the i2c-demux-pinctrl driver. It's a huge kludge with a number of
undesirable quirks. I don't know if the circumstances for adding
this I2C driver also applies for I3C, but it might be an argument
in favor of the proposed extra bus object...

Cheers,
Peter