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

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Date: Tue, 21 May 2024 10:34:33 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Sui Jingfeng <sui.jingfeng@...ux.dev>
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

On Thu, May 16, 2024 at 06:40:45PM GMT, Sui Jingfeng wrote:
> 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.

But *why*? What benefit does it bring that outweights the downsides I
listed earlier?

> 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.

Sure, we could use a function instead of another one. But again, what
benefit does that bring exactly?

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