linux-kernel - Re: [PATCH v4 4/4] i2c: Add multi-instantiate pseudo driver

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Date:   Wed, 8 Aug 2018 13:01:21 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Peter Rosin <peda@...ntia.se>
Cc:     Hans de Goede <hdegoede@...hat.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Darren Hart <dvhart@...radead.org>,
        Wolfram Sang <wsa@...-dreams.de>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Platform Driver <platform-driver-x86@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        John Garry <john.garry@...wei.com>,
        linux-i2c <linux-i2c@...r.kernel.org>
Subject: Re: [PATCH v4 4/4] i2c: Add multi-instantiate pseudo driver

On Wed, Aug 8, 2018 at 12:47 PM, Peter Rosin <peda@...ntia.se> wrote:
> On 2018-08-08 11:08, Andy Shevchenko wrote:
>> On Wed, Aug 8, 2018 at 11:30 AM, Hans de Goede <hdegoede@...hat.com> wrote:

>>> +       /* Count number of clients to instantiate */
>>> +       for (i = 0; inst_data[i].type; i++) {}
>>> +
>>> +       multi = devm_kmalloc(dev,
>>> +                       offsetof(struct i2c_multi_inst_data, clients[i]),
>>> +                       GFP_KERNEL);
>>> +       if (!multi)
>>> +               return -ENOMEM;
>>
>> Here I see the following:
>>  - it's kinda unusual use of offsetof(), perhaps i*sizeof() + sizeof()
>> would be more understandable
>>  - there is no guard against i == 0
>
> I don't see why a guard is needed?

Because there is no point to have a module loaded when there is none
client to serve.

> *Your* code below needs it, but that
> issue is not a concern for the original code.

I can admit that's not a big deal, just making logic slightly more robust.

> It might however be a
> good idea to fail the probe if there are no clients to instantiate, but
> that's a different issue...

That's what I have in mind.

>> multi = devm_kmalloc(sizeof(*multi), GFP_KERNEL);
>> if (!multi)
>>  return -ENOMEM;
>>
>> multi->clients = devm_kcalloc(i, sizeof(*multi->clients), GFP_KERNEL);
>> if (ZERO_PTR_OR_NULL(multi->clients))
>>  return -ENOMEM;
>>
>> But I would like to hear your (other's) opinion(s).
>
> I think using two allocations is a waste in this case.

On the other hand it makes code more readable. With offsetof() it is a
bit hard to get it on the first glance.

>>> +               if (inst_data[i].irq_idx != -1) {
>>
>>> = 0 sounds more robust
>
> But not as flexible/future-proof. Why should 0 be the only valid IRQ index?

Ah, because > is used usually is a quoting character in email you
missed the point.
It was written as >= 0.

-- 
With Best Regards,
Andy Shevchenko