Re: patch idea for wfx

["Fabio M. De Francesco" <fmdefrancesco@gmail.com>]

On mercoledì 6 aprile 2022 06:28:21 CEST Jaehee Park wrote:
> -----Section 2-----
> Here's my diff (below). I've implemented container_of() in function wfx_add_interface (to start).
> - If I'm understanding this correctly, we're replacing the line
> `struct wfx_vif *wvif = (struct wfx_vif *)vif->drv_priv`
> and replacing it with
> `struct wfx_vif *wvif = container_of(vif, struct ieee80211_vif, drv_priv)`
> 
> This is the error message I get:
> ./include/linux/container_of.h:17:41: error: initialization of ‘struct wfx_vif *’ from incompatible pointer type ‘struct ieee80211_vif *’ [-Werror=incompatible-pointer-types]

Hi Jaehee,

Probably, before changing any code which could be improved with the use of 
container_of(), you'd better see how this macro is used in other code that 
works properly.

Look at the following example. In drivers/dma/tegra20-apb-dma.c, we can look 
at a one line function called "to_tegra_dma_chan()":

static inline struct tegra_dma_channel *to_tegra_dma_chan(struct dma_chan *dc)
{
        return container_of(dc, struct tegra_dma_channel, dma_chan);
}

"to_tegra_dma_chan()" takes a pointer to a "struct dma_chan" that is called 
"dc" and it returns a pointer to the "struct tegra_dma_channel" which embeds 
the above-mentioned "struct dma_chan". 

Please take a look at the definition of "struct tegra_dma_channel". You'll 
notice that it has a field of type "dma_chan" that is called "dma_chan" 
(coincidentally the names of field and type are the same; this could confuse 
you but you easily see that they are different entities):

struct tegra_dma_channel {
        struct dma_chan dma_chan;
        ...
}

How can "to_tegra_dma_chan()" return a pointer to the embedding "struct 
tegra_dma_channel"?

It uses cointainer_of(). The first argument of that macro is the pointer 
passed to the function ("struct dma_chan *dc"). The second argument is 
the embedding "struct tegra_dma_channel". The third is the name of the 
field (not the type) pointed by "dc".

I hope it helps.

Fabio