Re: [PATCH v2 7/7] greybus: Add Rust UART node driver

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

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On Thu, 2026-08-27 at 13:24 +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 | 245 ++++++++++++++++++++++++++++++++++++++++
>  5 files changed, 310 insertions(+)
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index d047090be5f4..49c6dac72748 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -11339,6 +11339,7 @@ M:	Ayush Singh <ayush@beagleboard.com>
>  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 381d1a6ee135..34de913af287 100644
> --- a/drivers/greybus/Kconfig
> +++ b/drivers/greybus/Kconfig
> @@ -60,5 +60,20 @@ 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 RUST
> +	depends on GREYBUS_SOFTSVC
> +	depends on RUST_SERIAL_DEV_BUS_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..81151963c01e 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),) \
> +		$(obj)/gb_softsvc.o
> +endif
> +
> +# Output nova-core's crate metadata for use by nova-drm at compile time.
> +RUSTFLAGS_gb_softsvc.o += \
> +	--emit=metadata=$(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..3eb4f8ab3655
> --- /dev/null
> +++ b/drivers/greybus/gb_uart_node.rs
> @@ -0,0 +1,245 @@
> +// 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,
> +    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,
> +        })
> +    }
> +
> +    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;
> +                }
> +                HDLC_ESC => self.rx_in_esc = true,
> +                _ => {
> +                    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_buf.clear();
> +                        self.rx_in_esc = false;
> +                    }
> +                }
> +            }
> +        }
> +
> +        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>>,
(add me to CC please)

Instead of using a lock here, it might be a better idea to
synchronize/stop the receive callback before unbind is called in the
serdev rust abstraction. This would allow the abstraction to provide
mutable references to the driver data in `receive` and `unbind`. It
would also remove the Sync requirement.

I will send a patch soon.

Thanks
- Markus Probst

> +}
> +
> +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.
> +        let _ = this.rx.lock().take();
> +    }
> +}
> +
> +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",
> +}

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