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

[Ayush Singh <ayush@beagleboard.org>]

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.
>>
>> [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

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