Re: [PATCH v5 2/2] mailbox: mediatek: Add mtk-vcp-mailbox driver

Jjian Zhou (周建)
2025-09-23T11:47:03Z

On Tue, 2025-09-23 at 11:06 +0200, Nicolas Frattaroli wrote:
> External email : Please do not click links or open attachments until
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>
> On Tuesday, 23 September 2025 04:35:59 Central European Summer Time
> Jjian Zhou (周建) wrote:
> > On Mon, 2025-09-22 at 15:10 +0200, Nicolas Frattaroli wrote:
> > > External email : Please do not click links or open attachments
> > > until
> > > you have verified the sender or the content.
> > >
> > >
> > > On Monday, 22 September 2025 09:17:27 Central European Summer
> > > Time
> > > Jjian Zhou (周建) wrote:
> > > > On Sat, 2025-09-20 at 23:02 -0500, Jassi Brar wrote:
> > > > > External email : Please do not click links or open
> > > > > attachments
> > > > > until
> > > > > you have verified the sender or the content.
> > > > >
> > > > >
> > > > > On Fri, Sep 19, 2025 at 2:02 PM Nicolas Frattaroli
> > > > > <nicolas.frattaroli@collabora.com> wrote:
> > > > > >
> > > > > > On Friday, 19 September 2025 18:32:12 Central European
> > > > > > Summer
> > > > > > Time
> > > > > > Jassi Brar wrote:
> > > > > > > On Fri, Sep 19, 2025 at 3:31 AM Chen-Yu Tsai <
> > > > > > > wenst@chromium.org>
> > > > > > > wrote:
> > > > > > > >
> > > > > > > > On Fri, Sep 19, 2025 at 7:50 AM Jassi Brar <
> > > > > > > > jassisinghbrar@gmail.com> wrote:
> > > > > > > > >
> > > > > > > > > On Thu, Aug 21, 2025 at 9:12 PM Jjian Zhou <
> > > > > > > > > jjian.zhou@mediatek.com> wrote:
> > > > > > > > >
> > > > > > > > > .....
> > > > > > > > >
> > > > > > > > > > +#include <linux/module.h>
> > > > > > > > > > +#include <linux/of.h>
> > > > > > > > > > +#include <linux/platform_device.h>
> > > > > > > > > > +#include <linux/slab.h>
> > > > > > > > > > +
> > > > > > > > > > +struct mtk_vcp_mbox_priv {
> > > > > > > > >
> > > > > > > > > Maybe 'mtk_vcp_mbox' is a more appropriate name ?
> > > > > > > > >
> > > > > > > > > > + void __iomem *base;
> > > > > > > > > > + struct device *dev;
> > > > > > > > > > + struct mbox_controller mbox;
> > > > > > > > > > + const struct mtk_vcp_mbox_cfg *cfg;
> > > > > > > > > > + struct mtk_ipi_info ipi_recv;
> > > > > > > > >
> > > > > > > > > Maybe also have "struct mbox_chan chan[1]; " so that
> > > > > > > > > you
> > > > > > > > > don't have to
> > > > > > > > > allocate one during the probe.
> > > > > > > > > Also if you have "struct mbox_controller mbox;" as
> > > > > > > > > the
> > > > > > > > > first
> > > > > > > > > member,
> > > > > > > > > you could simply typecast that to get this structure.
> > > > > > > > > Something like "struct mpfs_mbox" in mailbox-mpfs.c
> > > > > > > >
> > > > > > > > I read somewhere that this way of subclassing is not
> > > > > > > > recommended.
> > > > > > > > Instead the base class should explicitly not be the
> > > > > > > > first
> > > > > > > > member.
> > > > > > > > And then container_of() should be used.
> > > > > > > >
> > > > > > > > I don't remember where I read this though. But I think
> > > > > > > > the
> > > > > > > > explicit
> > > > > > > > container_of() is easier for understanding the intent.
> > > > > > > >
> > > > > > >
> > > > > > > And how does container_of() work ? :)
> > > > > > > typcasting the first member to its parent is the simplest
> > > > > > > form of
> > > > > > > container_of.
> > > > > > >
> > > > > > > -j
> > > > > > >
> > > > > > >
> > > > > >
> > > > > > Which is why it's completely equivalent and since code is
> > > > > > supposed
> > > > > > to communicate meaning to humans, container_of should be
> > > > > > used.
> > > > > >
> > > > >
> > > > > Nobody is suggesting typecasting cfg, dev or anything else.
> > > > > Typecasting between mailbox controllers is fine and arguably
> > > > > easier
> > > > > on
> > > > > the eyes than using a container_of.
> > > > >
> > > > > -j
> > > >
> > > > OK. How about:
> > > > struct mtk_vcp_mbox *priv = (struct mtk_vcp_mbox *)chan-
> > > > > con_priv;
> > > >
> > > > Thanks.
> > > >
> > >
> > > An explicit cast would be worse, as at that point you're telling
> > > C to completely ignore any semblance of a type system it has.
> > >
> > >
> > >
> >
> > struct mtk_vcp_mbox *priv;
> > priv->dev = dev;
> > priv->chans[0].con_priv = priv;
> > The type of con_priv is "void *".
> > Would the conversion mentioned above also have the issue you
> > mentioned?
> >
> > Thanks.
> >
>
> No, in that case the cast is implicit. While void pointers do
> subvert the type system, they are needed in this case because
> the con_priv member needs to point at structs of any type.
>
> The problem is that when you do something like
>
> struct apple *a = something;
> struct orange *o = (struct orange *)a;
>
> then if the two structs (apple and orange) are incompatible,
> the compiler won't even yell at you, because you're explicitly
> casting.
>
> With an implicit cast:
>
> struct apple *a = something;
> struct orange *o = a;
>
> the compiler will tell you if you're doing something wrong.
> Here's a userspace code example to illustrate the point:
>
> #include <stdio.h>
>
> struct apple {
> const char *name;
> unsigned int weight;
> };
>
> struct orange {
> int x;
> int y;
> int z;
> };
>
> int main(int argc, char** argv)
> {
> struct apple a = {"Granny Smith", 200};
> // won't compile, good!
> /* struct orange *o = &a; */
> // will compile, bad!
> struct orange *o = (struct orange *)&a;
>
> printf("%d\n", o->x);
>
> return 0;
> }
>
> If you comment out the second struct orange line and uncomment the
> first, then you'll get a compilation error, which is what we want
> because the two structs are incompatible and we don't want the
> assignment to work in this case, as that would be a bug.
>
> The second struct orange line always compiles, even though the two
> structs are incompatible, and will cause nonsense to be printed.
>
> I hope this illustrates the point I was trying to make, which is
> that explicit casts make it harder to find issues because they
> force the language to simply accept the cast rather than give us
> a compilation error when something nonsensical is being done.
>
> Kind regards,
> Nicolas Frattaroli
>
>

I should not directly assign a variable to priv after an explicit
cast; instead, I should use implicit casting. This way, the compiler
can catch type mismatch errors, avoiding unexpected errors caused by
explicit casting.
Like:
static int mtk_vcp_mbox_send_data(struct mbox_chan *chan, void *data)
{
// should be
struct mtk_vcp_mbox *priv = chan->con_priv;
// should not

struct mtk_vcp_mbox *priv = (struct mtk_vcp_mbox *)chan->con_priv;