linux-kernel - Re: [PATCH v2 2/3] iommu/tegra-smmu: Rework .probe_device and .attach

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Date:   Wed, 30 Sep 2020 09:33:36 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     thierry.reding@...il.com, joro@...tes.org, krzk@...nel.org,
        vdumpa@...dia.com, jonathanh@...dia.com,
        linux-tegra@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] iommu/tegra-smmu: Rework .probe_device and
 .attach_dev

30.09.2020 09:13, Nicolin Chen пишет:
> On Wed, Sep 30, 2020 at 09:10:38AM +0300, Dmitry Osipenko wrote:
>> 30.09.2020 08:49, Nicolin Chen пишет:
>>> On Wed, Sep 30, 2020 at 08:11:52AM +0300, Dmitry Osipenko wrote:
>>>> 30.09.2020 03:30, Nicolin Chen пишет:
>>>>> +	/* An invalid mc pointer means mc and smmu drivers are not ready */
>>>>> +	if (IS_ERR_OR_NULL(mc))
>>>>
>>>> tegra_get_memory_controller() doesn't return NULL.
>>>
>>> Well, I don't want to assume that it'd do that forever, and the
>>> NULL check of IS_ERR_OR_NULL is marked "unlikely" so it doesn't
>>> hurt to have.
>>>
>>
>> I don't see any reasons why it won't do that forever.
>>
>> Secondly, public function can't be changed randomly without updating all
>> the callers.
>>
>> Hence there is no need to handle cases that can't ever happen and it
>> hurts readability of the code + original error code is missed.
> 
> I don't quite understand why an extra "_OR_NULL" would hurt
> readability....but I'd take a step back and use IS_ERR().
> 

The tegra_get_memory_controller() doesn't return NULL, hence the
NULL-check is misleading.

If I was reading that code for the first time and notice such a thing,
then instantly I'd have a much lower credibility to the whole code.