netdev - Re: [PATCH net-next v3 1/3] rust: core abstractions for network PHY drivers

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Message-ID: <4ced711a-009c-4e57-a758-1d13d97e18d2@lunn.ch>
Date: Mon, 9 Oct 2023 15:02:31 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: FUJITA Tomonori <fujita.tomonori@...il.com>, netdev@...r.kernel.org,
	rust-for-linux@...r.kernel.org, miguel.ojeda.sandonis@...il.com,
	greg@...ah.com, tmgross@...ch.edu
Subject: Re: [PATCH net-next v3 1/3] rust: core abstractions for network PHY
 drivers

On Mon, Oct 09, 2023 at 12:19:54PM +0000, Benno Lossin wrote:
> > +impl Device {
> > +    /// Creates a new [`Device`] instance from a raw pointer.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// For the duration of the lifetime 'a, the pointer must be valid for writing and nobody else
> > +    /// may read or write to the `phy_device` object.
> > +    pub unsafe fn from_raw<'a>(ptr: *mut bindings::phy_device) -> &'a mut Self {
> > +        unsafe { &mut *ptr.cast() }
> 
> Missing `SAFETY` comment.

Hi Benno

It is normal on Linux kernel mailing lists to trim the text to what is
just relevant to the reply. Comments don't get lost that way.

> > +    /// Returns true if the link is up.
> > +    pub fn get_link(&mut self) -> bool {
> 
> I would call this function `is_link_up`.

Please read the discussion on the previous versions of this patch. If
you still want to change the name, please give a justification.

> > +    /// Reads a given C22 PHY register.
> > +    pub fn read(&mut self, regnum: u16) -> Result<u16> {
> > +        let phydev = self.0.get();
> > +        // SAFETY: `phydev` is pointing to a valid object by the type invariant of `Self`.
> > +        // So an FFI call with a valid pointer.
> > +        let ret = unsafe {
> > +            bindings::mdiobus_read((*phydev).mdio.bus, (*phydev).mdio.addr, regnum.into())
> > +        };
> 
> Just a general question, all of these unsafe calls do not have
> additional requirements? So aside from the pointers being
> valid, there are no timing/locking/other safety requirements
> for calling the functions?

Locking has been discussed a number of times already. What do you mean
by timing?

> > +// SAFETY: `Registration` does not expose any of its state across threads.
> > +unsafe impl Send for Registration {}
> 
> Question: is it ok for two different threads to call `phy_drivers_register`
> and `phy_drivers_unregister`? If no, then `Send` must not be implemented.

The core phy_drivers_register() is thread safe. It boils down to a
driver_register() which gets hammered every kernel boot, so locking
issues would soon be found there.

> > +macro_rules! module_phy_driver {
> > +    (@replace_expr $_t:tt $sub:expr) => {$sub};
> > +
> > +    (@count_devices $($x:expr),*) => {
> > +        0usize $(+ $crate::module_phy_driver!(@replace_expr $x 1usize))*
> > +    };
> > +
> > +    (@device_table [$($dev:expr),+]) => {
> > +        #[no_mangle]
> > +        static __mod_mdio__phydev_device_table: [
> 
> Shouldn't this have a unique name? If we define two different
> phy drivers with this macro we would have a symbol collision?

I assume these are the equivalent of C static. It is not visible
outside the scope of this object file. The kernel has lots of tables
and they are mostly of very limited visibility scope, because only the
method registering/unregistering the table needs to see it.

       Andrew