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

On 9/4/26 2:19 AM, Markus Probst wrote:

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.

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: ARefserdev::Device,
• node: gb_softsvc::Module,

+}

+impl HdlcRx {

• fn new(sdev: ARefserdev::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: ARefserdev::Device,

+}

+impl GbNode {

• const fn new(sdev: ARefserdev::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

That sounds great. The lock here was basically only for getting a mut ref. I will base the next version on top of your patches.

I have added your email for the next patch version.

Best Regards,

Ayush Singh