linux-kernel - Re: [PATCH 0/2] drm/bridge: Add 'struct device *' field to the drm

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Date: Thu, 16 May 2024 18:40:45 +0800
From: Sui Jingfeng <sui.jingfeng@...ux.dev>
To: Maxime Ripard <mripard@...nel.org>
Cc: Neil Armstrong <neil.armstrong@...aro.org>,
 Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] drm/bridge: Add 'struct device *' field to the
 drm_bridge structure

Hi,

On 5/16/24 16:25, Maxime Ripard wrote:
> On Wed, May 15, 2024 at 11:19:58PM +0800, Sui Jingfeng wrote:
>> Hi,
>>
>>
>> On 5/15/24 22:58, Maxime Ripard wrote:
>>> On Wed, May 15, 2024 at 10:53:00PM +0800, Sui Jingfeng wrote:
>>>> On 5/15/24 22:30, Maxime Ripard wrote:
>>>>> On Wed, May 15, 2024 at 12:53:33AM +0800, Sui Jingfeng wrote:
>>>>>> On 2024/5/15 00:22, Maxime Ripard wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> On Tue, May 14, 2024 at 11:40:43PM +0800, Sui Jingfeng wrote:
>>>>>>>> Because a lot of implementations has already added it into their drived
>>>>>>>> class, promote it into drm_bridge core may benifits a lot. drm bridge is
>>>>>>>> a driver, it should know the underlying hardware entity.
>>>>>>> Is there some actual benefits, or is it theoretical at this point?
>>>>>>
>>>>>>
>>>>>> I think, DRM bridge drivers could remove the 'struct device *dev'
>>>>>> member from their derived structure. Rely on the drm bridge core
>>>>>> when they need the 'struct device *' pointer.
>>>>>
>>>>> Sure, but why do we need to do so?
>>>>>
>>>>> The other thread you had with Jani points out that it turns out that
>>>>> things are more complicated than "every bridge driver has a struct
>>>>> device anyway", it creates inconsistency in the API (bridges would have
>>>>> a struct device, but not other entities), and it looks like there's no
>>>>> use for it anyway.
>>>>>
>>>>> None of these things are deal-breaker by themselves, but if there's only
>>>>> downsides and no upside, it's not clear to me why we should do it at all.
>>>>
>>>> It can reduce boilerplate.
>>>
>>> You're still using a conditional here.
>>
>> It's for safety reason, prevent NULL pointer dereference.
>> drm bridge can be seen as either a software entity or a device driver.
>>
>> It's fine to pass NULL if specific KMS drivers intend to see
>> drm bridge as a pure software entity, and for internal use only.
>> Both use cases are valid.
> 
> Sorry, I don't follow you. We can't NULL dereference a pointer that
> doesn't exist.
> 
> Please state why we should merge this series: what does it fix or
> improve, aside from the potential gain of making bridges declare one
> less pointer in their private structure.

We could reduce more.

Bridge driver instances also don't have to embed 'struct i2c_client *'. 
We could use 'to_i2c_client(bridge->dev)' to retrieve the pointer,
where needed.

> Maxime

-- 
Best regards
Sui