netdev - Re: [PATCH net-next v7 2/5] rust: net::phy add module_phy

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Date: Mon, 20 Nov 2023 22:54:53 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: andrew@...n.ch
Cc: fujita.tomonori@...il.com, benno.lossin@...ton.me,
 boqun.feng@...il.com, netdev@...r.kernel.org,
 rust-for-linux@...r.kernel.org, tmgross@...ch.edu,
 miguel.ojeda.sandonis@...il.com, wedsonaf@...il.com
Subject: Re: [PATCH net-next v7 2/5] rust: net::phy add module_phy_driver
 macro

On Sun, 19 Nov 2023 16:50:34 +0100
Andrew Lunn <andrew@...n.ch> wrote:

> On Sun, Nov 19, 2023 at 06:25:44PM +0900, FUJITA Tomonori wrote:
>> On Fri, 17 Nov 2023 23:01:58 +0000
>> Benno Lossin <benno.lossin@...ton.me> wrote:
>> 
>> > On 11/17/23 23:54, Andrew Lunn wrote:
>> >> Each kernel module should be in its own symbol name space. The only
>> >> symbols which are visible outside of the module are those exported
>> >> using EXPORT_SYMBOL_GPL() or EXPORT_SYMBOL(). A PHY driver does not
>> >> export anything, in general.
>> >> 
>> >> Being built in also does not change this.
>> >> 
>> >> Neither drivers/net/phy/ax88796b_rust.o nor
>> >> rust/doctests_kernel_generated.o should have exported this symbol.
>> >> 
>> >> I've no idea how this actually works, i guess there are multiple
>> >> passes through the linker? Maybe once to resolve symbols across object
>> >> files within a module. Normal global symbols are then made local,
>> >> leaving only those exported with EXPORT_SYMBOL_GPL() or
>> >> EXPORT_SYMBOL()? A second pass through linker then links all the
>> >> exported symbols thorough the kernel?
>> > 
>> > I brought this issue up in [1], but I was a bit confused by your last
>> > reply there, as I have no idea how the `EXPORT_SYMBOL` macros work.
>> > 
>> > IIRC on the Rust side all public items are automatically GPL exported.
>> 
>> Hmm, they are public but doesn't look like exported by EXPORT_SYMBOL()
>> or EXPORT_SYMBOL_GPL().
> 
> Do they need to be public? Generally, a PHY driver does not export
> anything. So you can probably make them private. We just however need
> to ensure the compiler/linker does not think they are unused, so
> throws them away.

The Rust ax88796b driver doesn't export anything. The Rust and C
drivers handle the device_table in the same way when they are built as
a module.

$ grep __mod_mdio /proc/kallsyms
ffffffffa0358058 r __mod_mdio__phydev_device_table [ax88796b_rust]

$ grep __mod_mdio /proc/kallsyms
ffffffffa0288010 d __mod_mdio__asix_tbl_device_table	[ax88796b]

Note that when the Rust ax88796b driver is built-in,
__mod_mdio__phydev_device_table is not defined.

Sorry about my explanation, which leads to the confusion, I think.


> I would however like to get an understanding how EXPORT_SYMBOL* is
> supposed to work in rust. Can it really be hidden away? Or should
> methods be explicitly marked like C code? What is the Rust equivalent
> of the three levels of symbol scope we have in C?

If I understand correctly, there is no official way to export symbols
in Rust kernel modules; No equivalent to EXPORT_SYMBOL* for Rust code
built as a module yet.

Note that all core Rust functions are GPL exported so they are
available to Rust kernel modules.

`static` has a different meaning in Rust but I think that you can use
file scope and kernel-module scope in Rust.