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

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Message-ID: <20220504135059.7132b2b6@elisabeth>
Date:   Wed, 4 May 2022 13:50:59 +0200
From:   Stefano Brivio <sbrivio@...hat.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Jaehee <jhpark1013@...il.com>, Kalle Valo <kvalo@...nel.org>,
        Jérôme Pouiller <jerome.pouiller@...abs.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-staging@...ts.linux.dev,
        Outreachy Linux Kernel <outreachy@...ts.linux.dev>
Subject: Re: [PATCH] wfx: use container_of() to get vif

Hi Dan,

On Wed, 4 May 2022 12:33:48 +0300
Dan Carpenter <dan.carpenter@...cle.com> wrote:

> On Mon, May 02, 2022 at 02:10:07PM -0400, Jaehee wrote:
> > On Wed, Apr 20, 2022 at 7:58 AM Kalle Valo <kvalo@...nel.org> wrote:  
> > >
> > > Jaehee Park <jhpark1013@...il.com> writes:
> > >  
> > > > 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 function
> > > > wvif_to_vif(wvif) for container_of(), and replace all wvif->vif with
> > > > the newly defined container_of construct.
> > > >
> > > > Signed-off-by: Jaehee Park <jhpark1013@...il.com>  
> > >
> > > [...]
> > >  
> > > > +static inline struct ieee80211_vif *wvif_to_vif(struct wfx_vif *wvif)
> > > > +{
> > > > +     return container_of((void *)wvif, struct ieee80211_vif, drv_priv);
> > > > +}  
> > >
> > > Why the void pointer cast? Avoid casts as much possible.
> > >  
> > 
> > Hi Kalle,
> > 
> > Sorry for the delay in getting back to you about why the void pointer
> > cast was used.
> > 
> > In essence, I'm taking private data with a driver-specific pointer
> > and that needs to be resolved back to a generic pointer.
> > 
> > The private data (drv_priv) is declared as a generic u8 array in struct
> > ieee80211_vif, but wvif is a more specific type.
> > 
> > I wanted to also point to existing, reasonable examples such as:
> > static void iwl_mvm_tcm_uapsd_nonagg_detected_wk(struct work_struct *wk)
> > {
> >         struct iwl_mvm *mvm;
> >         struct iwl_mvm_vif *mvmvif;
> >         struct ieee80211_vif *vif;
> > 
> >         mvmvif = container_of(wk, struct iwl_mvm_vif,
> >                               uapsd_nonagg_detected_wk.work);
> >         vif = container_of((void *)mvmvif, struct ieee80211_vif, drv_priv);
> > 
> > in drivers/net/wireless$ less intel/iwlwifi/mvm/utils.c, which does the
> > same thing.
> > 
> > There are fifteen of them throughout:  
> 
> The cast is fine, but this email is frustrating.
> 
> It sounds like you are saying that you copied it from other code and
> that's not a good answer...  :/  It's easiest if you just copy and paste
> the build error and we can figure out why the cast is need for our
> selves...

...my bad, then.

I suggested to Jaehee she would *also* point out that there are already
a pile of usages (which I grepped for myself, by the way).

And that it's *obvious* that container_of() would trigger warnings
otherwise. Well, obvious just for me, it seems.

-- 
Stefano