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

[Markus Probst <markus.probst@posteo.de>]

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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.

> 
> Also, all of this information MUST be in the changelog text in order for
> us to be able to accept it.  You need to say _why_ a change is needed,
> not just _what_ the change does, as you know.
Will do.

Thanks
- Markus Probst

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