Re: [PATCH v3 2/4] serdev: add rust private data to serdev_device

[Markus Probst]

On Sat, 2026-03-14 at 14:31 +0100, Greg Kroah-Hartman wrote:
> On Sat, Mar 14, 2026 at 12:08:09PM +0000, Markus Probst wrote:
> > On Sat, 2026-03-14 at 12:52 +0100, Greg Kroah-Hartman wrote:
> > > On Sat, Mar 14, 2026 at 11:42:02AM +0000, Markus Probst wrote:
> > > > On Sat, 2026-03-14 at 09:07 +0100, Greg Kroah-Hartman wrote:
> > > > > On Fri, Mar 13, 2026 at 06:12:31PM +0000, Markus Probst wrote:
> > > > > > Add rust private data to `struct serdev_device`, as it is required by the
> > > > > > rust abstraction added in the following commit
> > > > > > (rust: add basic serial device bus abstractions).
> > > > > 
> > > > > why is rust "special" here?  What's wrong with the existing private
> > > > > pointer in this structure?  Why must we add another one?
> > > > Because in rust, the device drvdata will be set after probe has run. In
> > > > serdev, once the device has been opened, it can receive data. It must
> > > > be opened either inside probe or before probe, because it can only be
> > > > configured (baudrate, flow control etc.) and data written to after it
> > > > has been opened. Because it can receive data before drvdata has been
> > > > set yet, we need to ensure it waits on data receival for the probe to
> > > > be finished. Otherwise this would be a null pointer dereference. To do
> > > > this, we need to store a `Completion` for it to wait and a `bool` in
> > > > case the probe exits with an error. We cannot store this data in the
> > > > device drvdata, because this is where the drivers drvdata goes. We also
> > > > cannot create a wrapper of the drivers drvdata, because
> > > > `Device::drvdata::<T>()` would always fail in that case. That is why we
> > > > need a "rust_private_data" for this abstraction to store the
> > > > `Completion` and `bool`.
> > > 
> > > So why is this any different from any other bus type?  I don't see the
> > > "uniqueness" here that has not required this to happen for PCI or USB or
> > > anything else.
> > > 
> > > What am I missing?
> > In Short:
> > In serdev, we have to handle incoming device data (serdev calls on a
> > function pointer we provide in advance), even in the case that the
> > driver hasn't completed probe yet.
> 
> But how is that any different from a USB or PCI driver doing the same
> thing?  Why is serdev so unique here? 
> 
In PCI or USB we don't need to provide function pointers for callbacks
in advance, which will be can be called any time (even while probe).

> What specific serdev function
> causes this
> 
drivers/tty/serdev/serdev-ttyport.c basically only wraps the serdev
calls to tty calls. This isn't directly caused by a serdev function,
but by the tty part.

> why isn't it an issue with the C api?
> 
In C you can set the drvdata inside the probe and even with it not
being fully initialized.

With Rust the drvdata is only available after the probe.
But there is the posibility of serdev calling the provided callback
inside probe.

> Can we change the
> C code to not require this?
Serdev is very closely linked to tty.

There are 3 options:

1. Add a `rust_serdev_device_open` and `rust_serdev_device_ready`
function. `rust_serdev_device_open` would do the same thing as
`serdev_device_open`, but with calling `tty_buffer_lock_exclusive`
before opening the underlying tty port. `rust_serdev_device_ready`
would call `tty_buffer_unlock_exclusive`. Such functions would then
need to exist for every serdev controller (currently there is only
ttyport as serdev controller).

2. Rewrite parts of the tty subsystem.

Not sure what would need to be changed there yet. But this could also
affect the existing tty drivers, which are a lot in comparision to
serdev.

3. Keep the `rust_private_data` pointer in `serdev_device`.

This seems to be the simplest option to me.

Thanks
- Markus Probst

> 
> thanks,
> 
> greg k-h

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