Re: [PATCH v2 2/2] rust: miscdevice: access the `struct miscdevice` from fops->open()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Dec 09, 2024 at 12:07:13PM +0100, Danilo Krummrich wrote:
> On Mon, Dec 09, 2024 at 07:27:47AM +0000, Alice Ryhl wrote:
> > Providing access to the underlying `struct miscdevice` is useful for
> > various reasons. For example, this allows you access the miscdevice's
> > internal `struct device` for use with the `dev_*` printing macros.
> > 
> > Note that since the underlying `struct miscdevice` could get freed at
> > any point after the fops->open() call, only the open call is given
> > access to it. To print from other calls, they should take a refcount on
> > the device to keep it alive.
> > 
> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> > ---
> >  rust/kernel/miscdevice.rs | 19 ++++++++++++++++---
> >  1 file changed, 16 insertions(+), 3 deletions(-)
> > 
> > diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
> > index 0cb79676c139..c5af1d5ec4be 100644
> > --- a/rust/kernel/miscdevice.rs
> > +++ b/rust/kernel/miscdevice.rs
> > @@ -104,7 +104,7 @@ pub trait MiscDevice {
> >      /// Called when the misc device is opened.
> >      ///
> >      /// The returned pointer will be stored as the private data for the file.
> > -    fn open(_file: &File) -> Result<Self::Ptr>;
> > +    fn open(_file: &File, _misc: &MiscDeviceRegistration<Self>) -> Result<Self::Ptr>;
> 
> How is the user of this abstraction supposed to access the underlying struct
> miscdevice e.g. from other fops? AFAICS, there is no way for the user to store a
> device pointer / reference in their driver private data.

That should be "hung" off of the miscdevice structure.  In C that's done
through embedding the miscdevice structure within something else, don't
know how you all are going to do that in rust :)

Or, better yet, in your open callback, the rust code can set the file
private data pointer, that's what is done a lot as well, either could
work.

> I also think it's a bit weird to pass the registration structure in open() to
> access the device.

That's what the miscdevice api does today in C.  Well, it's embedded in
the file private pointer, so I guess just a function to call to get it
instead would work.

> I think we need an actual representation of a struct miscdevice, i.e.
> `misc::Device`.

I thought we have that already?

thanks,

greg k-h