linux-kernel - Re: [PATCH] staging: wfx: use container

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Date:   Sat, 09 Apr 2022 15:54:09 +0200
From:   Jérôme Pouiller <jerome.pouiller@...abs.com>
To:     Julia Lawall <julia.lawall@...ia.fr>,
        Jaehee Park <jhpark1013@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org,
        outreachy@...ts.linux.dev, Stefano Brivio <sbrivio@...hat.com>
Subject: Re: [PATCH] staging: wfx: use container_of() to get vif

On Saturday 9 April 2022 00:21:02 CEST Jaehee Park wrote:
> On Fri, Apr 08, 2022 at 11:49:49AM +0200, Julia Lawall wrote:
> > On Thu, 7 Apr 2022, Jaehee Park wrote:
> >
> > > Use container_of() to get vif. This improves the code in two ways:
> > > [1] it speeds up the compilation because container_of() saves steps to
> > > retrieve vif (the representation of ieee80211_vif), and
> >
> > I don't understand this part.
> >
> > julia
> >
> 
> Thank you for your message. I revised the commit message (below)--
> I hope it makes it clearer. I'm open to suggestions as well!
> 
> Currently, upon virtual interface creation, wfx_add_interface() stores
> a reference to the corresponding struct ieee80211_vif in private data,
> for later usage. This is not needed when using the container_of
> construct. This construct already has all the info it needs to retrieve
> the reference to the corresponding struct from the offset that is
> already available, inherent in container_of(), between its type and
> member inputs (struct ieee80211_vif and drv_priv, respectively).
> Remove vif (which was previously storing the reference to the struct
> ieee80211_vif) from the struct wfx_vif, define a macro
> wvif_to_vif(wvif) for container_of(), and replace all wvif->vif with
> the newly defined container_of construct.
> 
> Sorry if I'm putting out more information than was asked for:
> Diagram is worth a thousand words; this one is made by Stefano when
> he was explaining the container_of() construct:
> 
> .-----------------------------------.
> |  .------------------------------. |
> '->| struct ieee80211_vif         | |
>    |------------------------------| |
>    | 1                            | |
>    | 2                            | |
>    |  .--------------------------.| |
>    | 3| struct wfx_vif drv_priv   | |
>    |  |---------------------------| |
>    |  | ...                       | |
>    |  |                           | |
>    |  | struct ieee80211_vif *vif---'
>    '------------------------------'
> In this example, the offset is 3.
> What I want to convey here is the compiler already knows the offset
> of drv_priv within struct ieee80211_vif by using the container_of()
> construct. So, it doesn't need to do extra work.

Wow, nice ascii art :)

I think that Julia has not understand why you said it "speeds up the
compilation". I think you meant "speeds up the execution", don't you?



-- 
Jérôme Pouiller