Re: [PATCH v3 7/7] greybus: Add Rust UART node driver

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

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On Tue, 2026-10-13 at 21:59 +0530, Ayush Singh wrote:
> Markus Probst <markus.probst@posteo.de> writes:
> 
> > On Tue, 2026-10-13 at 17:30 +0530, Ayush Singh wrote:
> > > Add a driver for Greybus nodes attached over a plain serial port. The
> > > node is registered with the software SVC (gb-softsvc), which handles the
> > > SVC protocol on behalf of the AP, so no dedicated coprocessor running
> > > SVC firmware is needed.
> > > 
> > > Greybus messages are carried over HDLC framing on the wire. Each frame
> > > carries a one-byte address (0x01 for Greybus) and control byte, followed
> > > by the 16-bit CPort ID and the Greybus message itself.
> > > 
> > > Port parameters are taken from the firmware node: "baudrate" if
> > > present, otherwise 115200, with flow control and parity disabled.
> > > 
> > > Since gb-uart-node imports types from gb-softsvc, Rust to Rust calling
> > > setup from nova-core [0] is being used.
> > > 
> > > [0]: https://lore.kernel.org/all/20260622-nova-exports-v5-0-6191773fc977@nvidia.com/
> > > 
> > > Signed-off-by: Ayush Singh <ayush@beagleboard.org>
> > > ---
> > >  MAINTAINERS                     |   1 +
> > >  drivers/greybus/.gitignore      |   1 +
> > >  drivers/greybus/Kconfig         |  15 +++
> > >  drivers/greybus/Makefile        |  48 ++++++++
> > >  drivers/greybus/gb_uart_node.rs | 253 ++++++++++++++++++++++++++++++++++++++++
> > >  5 files changed, 318 insertions(+)
> > > 
> > > diff --git a/MAINTAINERS b/MAINTAINERS
> > > index 521bc6c67979..5bdd500ef63e 100644
> > > --- a/MAINTAINERS
> > > +++ b/MAINTAINERS
> > > @@ -11527,6 +11527,7 @@ M:	Ayush Singh <ayush@beagleboard.org>
> > >  L:	greybus-dev@lists.linaro.org (moderated for non-subscribers)
> > >  S:	Maintained
> > >  F:	Documentation/devicetree/bindings/beagle/beagle,beagleconnect-freedom.yaml
> > > +F:	drivers/greybus/gb_uart_node.rs
> > >  
> > >  GREYBUS SUBSYSTEM
> > >  M:	Johan Hovold <johan@kernel.org>
> > > diff --git a/drivers/greybus/.gitignore b/drivers/greybus/.gitignore
> > > new file mode 100644
> > > index 000000000000..ff9c4a3539b4
> > > --- /dev/null
> > > +++ b/drivers/greybus/.gitignore
> > > @@ -0,0 +1 @@
> > > +exports_gb_softsvc_generated.h
> > > diff --git a/drivers/greybus/Kconfig b/drivers/greybus/Kconfig
> > > index 709563f8c0ad..ce2030161f30 100644
> > > --- a/drivers/greybus/Kconfig
> > > +++ b/drivers/greybus/Kconfig
> > > @@ -61,5 +61,20 @@ config GREYBUS_SOFTSVC
> > >  	  To compile this code as a module, choose M here: the module
> > >  	  will be called gb-softsvc.ko
> > >  
> > > +config GREYBUS_UART_NODE
> > > +	tristate "Greybus UART node transport"
> > > +	depends on GREYBUS_SOFTSVC
> > > +	depends on RUST_SERIAL_DEV_BUS_ABSTRACTIONS
> > > +	depends on RUST_GREYBUS_ABSTRACTIONS
> > > +	select RUST_CRC_CCITT_ABSTRACTIONS
> > > +	help
> > > +	  Select this option if you have a Greybus node connected over a
> > > +	  serial port. The node is registered with the software SVC, which
> > > +	  handles the SVC protocol on behalf of the AP, so no dedicated
> > > +	  coprocessor running SVC firmware is required.
> > > +
> > > +	  To compile this code as a module, choose M here: the module
> > > +	  will be called gb-uart-node.ko
> > > +
> > >  endif	# GREYBUS
> > >  
> > > diff --git a/drivers/greybus/Makefile b/drivers/greybus/Makefile
> > > index e6f594128802..bf4d08f728fe 100644
> > > --- a/drivers/greybus/Makefile
> > > +++ b/drivers/greybus/Makefile
> > > @@ -28,3 +28,51 @@ obj-$(CONFIG_GREYBUS_ES2)	+= gb-es2.o
> > >  obj-$(CONFIG_GREYBUS_SOFTSVC)	+= gb-softsvc.o
> > >  gb-softsvc-y			+= gb_softsvc.o gb_softsvc_exports.o
> > >  
> > > +obj-$(CONFIG_GREYBUS_UART_NODE)	+= gb-uart-node.o
> > > +gb-uart-node-y			+= gb_uart_node.o
> > > +
> > > +# Export Rust symbols from gb-softsvc only if gb-uart-node actually references them.
> > > +gb-softsvc-export-deps := $(if $(CONFIG_GREYBUS_UART_NODE),$(obj)/gb_uart_node.o)
> > > +
> > > +rust_needed_exports = \
> > > +	{ $(if $(strip $(2)),$(NM) -u $(2);,) echo "__DEFINED_RUST_SYMBOLS__"; \
> > > +	  $(NM) -p --defined-only $(1); } | \
> > > +	awk -v fmt='$(3)' ' \
> > > +		/^__DEFINED_RUST_SYMBOLS__$$/ { defs = 1; next } \
> > > +		!defs { if ($$NF ~ /^_R/) needed[$$NF] = 1; next } \
> > > +		defs && $$2 ~ /(T|R|D|B)/ && $$3 ~ /^_R/ && \
> > > +			$$3 !~ /_(init|cleanup)_module$$/ && \
> > > +			$$3 !~ /__(pfx|cfi|odr_asan)/ && \
> > > +			$$3 in needed { printf fmt, $$3 } \
> > > +	'
> > > +
> > > +quiet_cmd_exports = EXPORTS $@
> > > +      cmd_exports = \
> > > +	$(call rust_needed_exports,$<,$(gb-softsvc-export-deps),EXPORT_SYMBOL_RUST_GPL(%s);\n) > $@
> > > +
> > > +$(obj)/exports_gb_softsvc_generated.h: $(obj)/gb_softsvc.o $(gb-softsvc-export-deps) FORCE
> > > +	$(call if_changed,exports)
> > > +
> > > +targets += exports_gb_softsvc_generated.h
> > > +
> > > +$(obj)/gb_softsvc_exports.o: $(obj)/exports_gb_softsvc_generated.h
> > > +CFLAGS_gb_softsvc_exports.o := -I $(objtree)/$(obj)
> > > +
> > > +ifdef CONFIG_MODVERSIONS
> > > +# The C export shim declares Rust symbols as `extern int`, so reuse its export
> > > +# list but generate symbol CRCs from the Rust object instead of the shim's DWARF.
> > > +$(obj)/gb_softsvc_exports.o: private cmd_gensymtypes_c = \
> > > +	$(call getexportsymbols,\1) | \
> > > +	$(objtree)/scripts/gendwarfksyms/gendwarfksyms \
> > > +		$(if $(KBUILD_GENDWARFKSYMS_STABLE), --stable) \
> > > +		$(if $(KBUILD_SYMTYPES), --symtypes $(@:.o=.symtypes),) \
> > > +		$(obj)/gb_softsvc.o
> > > +endif
> > > +
> > > +# Output gb_softsvc's crate metadata for use by gb_uart_node at compile time.
> > > +RUSTFLAGS_gb_softsvc.o += \
> > > +	--emit=metadata=$(objtree)/$(obj)/libgb_softsvc.rmeta
> > > +
> > > +# Allow nova-drm to import nova-core's types.
> > > +$(obj)/gb_uart_node.o: $(obj)/gb_softsvc.o
> > > +RUSTFLAGS_gb_uart_node.o := -L $(objtree)/$(obj) --extern gb_softsvc
> > > diff --git a/drivers/greybus/gb_uart_node.rs b/drivers/greybus/gb_uart_node.rs
> > > new file mode 100644
> > > index 000000000000..f719ee35e8f3
> > > --- /dev/null
> > > +++ b/drivers/greybus/gb_uart_node.rs
> > > @@ -0,0 +1,253 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +
> > > +//! Greybus UART Node driver
> > > +
> > > +use kernel::{
> > > +    alloc::Flags,
> > > +    crc_ccitt::crc_ccitt,
> > > +    device::{
> > > +        AsBusDevice,
> > > +        Bound,
> > > +        Core, //
> > > +    },
> > > +    error::code,
> > > +    new_spinlock,
> > > +    of, //
> > > +    prelude::*,
> > > +    serdev,
> > > +    sync::{
> > > +        aref::ARef,
> > > +        Arc,
> > > +        SpinLock, //
> > > +    },
> > > +};
> > > +
> > > +use zerocopy::little_endian;
> > > +use zerocopy_derive::{FromBytes, Immutable, KnownLayout};
> > > +
> > > +const HDLC_MAX_FRAME_LEN: usize = 256;
> > > +
> > > +const HDLC_FRAME: u8 = 0x7E;
> > > +const HDLC_ESC: u8 = 0x7D;
> > > +const HDLC_XOR: u8 = 0x20;
> > > +const HDLC_EXPECTED_CRC: u16 = 0xf0b8;
> > > +
> > > +const ADDRESS_GREYBUS: u8 = 0x01;
> > > +
> > > +#[repr(C, packed)]
> > > +#[derive(FromBytes, Immutable, KnownLayout)]
> > > +struct GreybusFrame {
> > > +    cport: little_endian::U16,
> > > +    msg: [u8],
> > > +}
> > > +
> > > +struct HdlcRx {
> > > +    rx_buf: KVec<u8>,
> > > +    rx_in_esc: bool,
> > > +    rx_overflow: bool,
> > > +    sdev: ARef<serdev::Device>,
> > > +    node: gb_softsvc::Module,
> > > +}
> > > +
> > > +impl HdlcRx {
> > > +    fn new(sdev: ARef<serdev::Device>, node: gb_softsvc::Module) -> Result<Self> {
> > > +        Ok(Self {
> > > +            node,
> > > +            sdev,
> > > +            rx_buf: KVec::with_capacity(HDLC_MAX_FRAME_LEN, GFP_KERNEL)?,
> > > +            rx_in_esc: false,
> > > +            rx_overflow: false,
> > > +        })
> > > +    }
> > > +
> > > +    fn frame_finish(&self) -> Result<()> {
> > > +        if self.rx_buf.len() < 4 {
> > > +            return Err(code::EFAULT);
> > > +        }
> > > +
> > > +        let crc = crc_ccitt(0xffff, &self.rx_buf);
> > > +        if crc != HDLC_EXPECTED_CRC {
> > > +            dev_warn!(self.sdev.as_ref(), "CRC failed {}", crc);
> > > +            return Ok(());
> > > +        }
> > > +
> > > +        let addr = self.rx_buf[0];
> > > +        let _ctrl = self.rx_buf[1];
> > > +        let payload = &self.rx_buf[2..self.rx_buf.len() - size_of::<u16>()];
> > > +
> > > +        match addr {
> > > +            ADDRESS_GREYBUS => {
> > > +                let frame = GreybusFrame::ref_from_bytes(payload).map_err(|_| code::EINVAL)?;
> > > +                self.node.submit_message(0, frame.cport.into(), &frame.msg)
> > > +            }
> > > +            _ => Err(code::EINVAL),
> > > +        }
> > > +    }
> > > +
> > > +    fn rx(&mut self, data: &[u8]) -> usize {
> > > +        for i in data.iter() {
> > > +            match *i {
> > > +                HDLC_FRAME => {
> > > +                    if !self.rx_buf.is_empty() {
> > > +                        if let Err(e) = self.frame_finish() {
> > > +                            dev_warn!(self.sdev.as_ref(), "bad frame: {e:?}\n");
> > > +                        }
> > > +                    }
> > > +
> > > +                    self.rx_buf.clear();
> > > +                    self.rx_in_esc = false;
> > > +                    self.rx_overflow = false;
> > > +                }
> > > +                HDLC_ESC => self.rx_in_esc = true,
> > > +                // Skip data if buffer has overflown.
> > > +                _ if self.rx_overflow => {}
> > > +                _ => {
> > > +                    let c = if self.rx_in_esc { *i ^ HDLC_XOR } else { *i };
> > > +                    self.rx_in_esc = false;
> > > +
> > > +                    if self.rx_buf.push_within_capacity(c).is_err() {
> > > +                        dev_warn!(self.sdev.as_ref(), "buffer overflow. Dropping frame");
> > > +
> > > +                        self.rx_overflow = true;
> > > +                        self.rx_buf.clear();
> > > +                    }
> > > +                }
> > > +            }
> > > +        }
> > > +
> > > +        data.len()
> > > +    }
> > > +}
> > > +
> > > +struct GbNode {
> > > +    sdev: ARef<serdev::Device>,
> > > +}
> > > +
> > > +impl GbNode {
> > > +    const fn new(sdev: ARef<serdev::Device>) -> Self {
> > > +        Self { sdev }
> > > +    }
> > > +
> > > +    fn fill_buf(mut crc: u16, data: &[u8], buf: &mut KVec<u8>) -> Result<u16> {
> > > +        for i in data {
> > > +            crc = crc_ccitt(crc, &[*i]);
> > > +            if *i == HDLC_ESC || *i == HDLC_FRAME {
> > > +                buf.push_within_capacity(HDLC_ESC)?;
> > > +                buf.push_within_capacity(i ^ HDLC_XOR)?;
> > > +            } else {
> > > +                buf.push_within_capacity(*i)?;
> > > +            }
> > > +        }
> > > +
> > > +        Ok(crc)
> > > +    }
> > > +}
> > > +
> > > +impl gb_softsvc::InterfaceOps for GbNode {
> > > +    fn write(&self, data: &[u8], cport: u16, gfp_mask: Flags) -> Result<()> {
> > > +        // SAFETY: `GbNode` only exists while its serdev driver is bound, so the device is in the
> > > +        // `Bound` state for the duration of this call.
> > > +        let bound: &serdev::Device<Bound> =
> > > +            unsafe { serdev::Device::from_device(self.sdev.as_ref().as_bound()) };
> > > +
> > > +        let mut buf = KVec::with_capacity(HDLC_MAX_FRAME_LEN, gfp_mask)?;
> > > +
> > > +        let mut crc = 0xffff;
> > > +
> > > +        buf.push_within_capacity(HDLC_FRAME)?;
> > > +
> > > +        crc = Self::fill_buf(crc, &[ADDRESS_GREYBUS, 0x03], &mut buf)?;
> > > +        crc = Self::fill_buf(crc, &cport.to_le_bytes(), &mut buf)?;
> > > +        crc = Self::fill_buf(crc, data, &mut buf)?;
> > > +
> > > +        crc ^= 0xffff;
> > > +        Self::fill_buf(crc, &crc.to_le_bytes(), &mut buf)?;
> > > +
> > > +        buf.push_within_capacity(HDLC_FRAME)?;
> > > +
> > > +        bound.write_all(&buf, 0)?;
> > > +
> > > +        Ok(())
> > > +    }
> > > +}
> > > +
> > > +#[pin_data]
> > > +struct GbUartNode {
> > > +    #[pin]
> > > +    rx: SpinLock<Option<HdlcRx>>,
> > > +}
> > > +
> > > +impl GbUartNode {
> > > +    fn init(sdev: &serdev::Device<Core<'_>>) -> Result<HdlcRx> {
> > > +        if sdev
> > > +            .set_baudrate(
> > > +                sdev.as_ref()
> > > +                    .fwnode()
> > > +                    .and_then(|fwnode| fwnode.property_read(c"baudrate").optional())
> > > +                    .unwrap_or(115200),
> > > +            )
> > > +            .is_err()
> > > +        {
> > > +            return Err(EINVAL);
> > > +        }
> > > +        sdev.set_flow_control(false);
> > > +        sdev.set_parity(serdev::Parity::None)?;
> > > +
> > > +        let node = gb_softsvc::Module::new(&[Arc::new(GbNode::new(sdev.into()), GFP_KERNEL)?])?;
> > > +
> > > +        HdlcRx::new(sdev.into(), node)
> > > +    }
> > > +}
> > > +
> > > +kernel::of_device_table!(
> > > +    OF_TABLE,
> > > +    <GbUartNode as serdev::Driver>::IdInfo,
> > > +    [(of::DeviceId::new(c"beagle,beagleconnect-freedom"), ())]
> > > +);
> > > +
> > > +#[vtable]
> > > +impl serdev::Driver for GbUartNode {
> > > +    type IdInfo = ();
> > > +    type Data<'bound> = Self;
> > > +    const OF_ID_TABLE: Option<of::IdTable<Self::IdInfo>> = Some(&OF_TABLE);
> > > +
> > > +    fn probe<'bound>(
> > > +        sdev: &'bound serdev::Device<Core<'_>>,
> > > +        _info: Option<&'bound Self::IdInfo>,
> > > +    ) -> impl PinInit<Self, Error> + 'bound {
> > > +        dev_dbg!(sdev.as_ref(), "Probe gb_uart_node.\n");
> > > +
> > > +        try_pin_init!(Self {
> > > +            rx <- new_spinlock!(Some(Self::init(sdev)?), "gb_uart_node::rx"),
> > > +        }? Error)
> > > +    }
> > > +
> > > +    fn receive<'bound>(
> > > +        _sdev: &'bound serdev::Device<Bound>,
> > > +        this: Pin<&Self>,
> > > +        data: &[u8],
> > > +    ) -> usize {
> > > +        if let Some(mut guard) = this.rx.try_lock() {
> > > +            if let Some(ref mut hdlc_rx) = *guard {
> > > +                return hdlc_rx.rx(data);
> > > +            }
> > > +        }
> > > +
> > > +        0
> > > +    }
> > > +
> > > +    fn unbind<'bound>(_: &'bound serdev::Device<Core<'_>>, this: Pin<&Self::Data<'bound>>) {
> > > +        // Getting a bound device is not possible after this point. So drop HdlcRx.
> > > +        // This also drops HdlcRx::node, which should ensure that GbNode::write will not be called
> > > +        // after this point.
> > > +        let _ = this.rx.lock().take();
> > Not exactly.
> > 
> > e.g.
> > ```
> > struct SerdevDriver<'bound> {
> >   dev: &'bound serdev::Device<Bound>,
> > }
> > 
> > impl<'bound> Drop for SerdevDriver<'bound> {
> > 
> >   fn drop(&mut self) {
> >     // Access to a bound device should still be possible here.
> >   }
> > 
> > }
> > ```
> > 
> > So afaik there should be no advantage of manually dropping it here.
> > Drop of `GbUartNode` should automatically drop the spinlock and the
> > data contained. The bound serial device is still valid at this point.
> > 
> > This would likely make the usage of Option<> obsolete too.
> > 
> > Thanks
> > - Markus Probst
> 
> My understanding was that the state of serdev device is unknown between
> unbind and Drop calls. That would make the unsafe Bound device conversion
> in GbNode::write unsound.
> 
> If I don't need to do the early drop, yes, that would remove the Option
> which would be great.

.write is still valid inside Drop.

Currently there is a small difference between unbind and Drop though
(that shouldn't matter here):

while unbind, data can still be received.
while Drop, data can only be sent. No data will be received (Drop and
receive never runs concurrently).

Thanks
- Markus Probst


> 
> > 
> > > +    }
> > > +}
> > > +
> > > +kernel::module_serdev_device_driver! {
> > > +    type: GbUartNode,
> > > +    name: "gb_uart_node",
> > > +    authors: ["Ayush Singh <ayush@beagleboard.org>"],
> > > +    description: "Greybus node connected over UART",
> > > +    license: "GPL v2",
> > > +}
> > 
> 
> 
> Best Regards,
> Ayush Singh

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