linux-kernel - Re: [PATCH v8] i2c: virtio: add a virtio i2c frontend driver

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Date:   Fri, 19 Mar 2021 14:29:50 +0800
From:   Jie Deng <jie.deng@...el.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        "Enrico Weigelt, metux IT consult" <lkml@...ux.net>,
        Linux I2C <linux-i2c@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Wolfram Sang <wsa@...nel.org>,
        Jason Wang <jasowang@...hat.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        conghui.chen@...el.com, kblaiech@...lanox.com,
        jarkko.nikula@...ux.intel.com,
        Sergey Semin <Sergey.Semin@...kalelectronics.ru>,
        Mike Rapoport <rppt@...nel.org>, loic.poulain@...aro.org,
        Tali Perry <tali.perry1@...il.com>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        yu1.wang@...el.com, shuo.a.liu@...el.com,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v8] i2c: virtio: add a virtio i2c frontend driver


On 2021/3/19 13:40, Viresh Kumar wrote:
> On 19-03-21, 13:31, Jie Deng wrote:
>> On 2021/3/19 11:54, Viresh Kumar wrote:
>>> On 18-03-21, 15:52, Arnd Bergmann wrote:
>>>> Allowing multiple virtio-i2c controllers in one system, and multiple i2c
>>>> devices attached to each controller is clearly something that has to work.
>>> Good.
>>>
>>>> I don't actually see a limitation though. Viresh, what is the problem
>>>> you see for having multiple controllers?
>>> I thought this would be a problem in that case as we are using the global
>>> virtio_adapter here.
>>>
>>> +       vi->adap = &virtio_adapter;
>>> +       i2c_set_adapdata(vi->adap, vi);
>>>
>>> Multiple calls to probe() will end up updating the same pointer inside adap.
>>>
>>> +       vi->adap->dev.parent = &vdev->dev;
>>>
>>> Same here, overwrite.
>>>
>>> +       /* Setup ACPI node for controlled devices which will be probed through ACPI */
>>> +       ACPI_COMPANION_SET(&vi->adap->dev, ACPI_COMPANION(pdev));
>>> +       vi->adap->timeout = HZ / 10;
>>>
>>> These may be fine, but still not ideal I believe.
>>>
>>> +       ret = i2c_add_adapter(vi->adap);
>>> i
>>> This should be a problem as well, we must be adding this to some sort of list,
>>> doing some RPM stuff, etc ?
>>>
>>> Jie, the solution is to allocate memory for adap at runtime in probe and remove
>>> the virtio_adapter structure completely.
>>
>> If you want to support that. Then I think we don't need to change the
>> following at all.
>>
>>> +    .algo = &virtio_algorithm,
>>> +
>>> +        return ret;
>>> +
>>> +    vi->adap = virtio_adapter;
>> This is strange, why are you allocating memory for adapter twice ?
>> Once for virtio_adapter and once for vi->adap ? Either fill the fields
>> directly for v->adap here and remove virtio_adapter or make vi->adap a
>> pointer.
> Yes, your previous version was partly okay but you don't need the
> virtio_algorithm structure to be allocated. There are only 4 fields you are
> updating here, just fill them directly in vi->adap.
>
> (FWIW, I also suggested the same when I said
> "Either fill the fields directly for v->adap here and remove virtio_adapter".
> )
>
> See how drivers/i2c/busses/i2c-versatile.c and most of the other drivers have
> done it.
I also see example drivers/i2c/busses/i2c-xiic.c. Some people might 
think this way is more clearer than

updating each member in probe. Basically, I think it's just a matter of 
personal preference which doesn't

solve any problems.