On Wed, Nov 20, 2024 at 9:07 PM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
>
> On Wed, Nov 20, 2024 at 02:50:01PM +0000, Alice Ryhl wrote:
> > Using the vma support introduced by the previous commit, introduce mmap
> > support for miscdevices. The mmap call is given a vma that is undergoing
> > initial setup, so the VmAreaNew type is used.
>
> Again, I'd be explicit about the VMA being passed to a struct
> file_operations->mmap() hook on mmap. Otherwise this seems super vague to
> me!
I wasn't sure if vmas could be constructed in any context other than
mmap. Will reword.
> > Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> > ---
> > rust/kernel/miscdevice.rs | 28 ++++++++++++++++++++++++++++
> > 1 file changed, 28 insertions(+)
> >
> > diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
> > index 7e2a79b3ae26..4e4b9476e092 100644
> > --- a/rust/kernel/miscdevice.rs
> > +++ b/rust/kernel/miscdevice.rs
> > @@ -11,6 +11,7 @@
> > use crate::{
> > bindings,
> > error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR},
> > + mm::virt::VmAreaNew,
> > prelude::*,
> > str::CStr,
> > types::{ForeignOwnable, Opaque},
> > @@ -110,6 +111,11 @@ fn release(device: Self::Ptr) {
> > drop(device);
> > }
> >
> > + /// Handle for mmap.
> > + fn mmap(_device: <Self::Ptr as ForeignOwnable>::Borrowed<'_>, _vma: &VmAreaNew) -> Result {
>
> You will have to help me with this :) ForeignOwnable, Borrowed<'_>, _vma I'm a
> bit lost here!
When you implement the Miscdevice for your own type, you write a block
like this one:
impl Miscdevice for MyType {
type Ptr = Arc<MyType>;
...
}
Here Arc is a pointer type (very similar to ARef) that represents
ownership over a refcount to a refcounted value.
In this case:
* Self refers to MyType.
* Self::Ptr refers to Arc<MyType>
* <Self::Ptr as ForeignOwnable>::Borrowed<'_> refers to ArcBorrow<MyType>
The last step is resolved to ArcBorrow<MyType> because of this impl
block in rust/kernel/sync/arc.rs:
impl<T: 'static> ForeignOwnable for Arc<T> {
type Borrowed<'a> = ArcBorrow<'a, T>;
...
}
Note that Self::Ptr is short-hand for <Self as Miscdevice>::Ptr.
> > + kernel::build_error(VTABLE_DEFAULT_ERROR)
>
> What is this? Is this not yet implemented or something, and this is a
> placeholder or something?
It's a build-time assertion that this function is dead-code eliminated.
There are two cases:
* Either the driver does not override mmap. In this case, we store a
null pointer in the fops table.
* Or the driver overrides mmap with their own implementation. In this
case, we store a function pointer to whichever implementation they
provided.
In neither case is the above function present anywhere in the final executable.
Alice
