Re: [PATCH RFC 05/10] ARM: debug: Add Cortina Systems CS75xx UART0 support

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] arm: debug: cs75xx senduart clobbers register on BE8 breaking newlines
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commit 8bcfce484cc05ad1abae605952a7f9c8f05d1c26
Author: Fil Dunsky <filipp.dunsky@gmail.com>

ARM: debug: Add Cortina Systems CS75xx UART0 support

Add low-level debug output through UART0 of the Cortina Systems CS75xx
SoCs. The UART is mapped at a fixed virtual address by the generic
debug_ll_io_init(), so no platform map_io is needed.

> diff --git a/arch/arm/include/debug/cs75xx.S b/arch/arm/include/debug/cs75xx.S
> new file mode 100644
> index 0000000000000..d840384cc32bd
> --- /dev/null
> +++ b/arch/arm/include/debug/cs75xx.S

[ ... ]

> +		.macro	addruart, rp, rv, tmp
> +		ldr	\rp, =CONFIG_DEBUG_UART_PHYS
> +		ldr	\rv, =CONFIG_DEBUG_UART_VIRT
> +		.endm
> +
> +		.macro	senduart, rd, rx
> +		ARM_BE8(rev \rd, \rd)
> +		str	\rd, [\rx, #CS75XX_UART_TXDAT]
> +		.endm

[Severity: Medium]
Does this rev instruction clobber the character register on big-endian (BE-8)
builds?

The macro reverses the bytes of \rd but does not restore it via a second rev.
This violates the caller's expectation that the character register is
preserved.

For example, when the compressed bootloader's puts function sends a newline,
it relies on the register retaining its value:

arch/arm/boot/compressed/head.S:puts()
    2:      writeb  r2, r3, r1
            mov     r1, #0x00020000
    3:      subs    r1, r1, #1
            bne     3b
            teq     r2, #'\n'
            moveq   r2, #'\r'
            beq     2b

If writeb (which wraps senduart) permanently swaps r2 to 0x0A000000, the teq
comparison fails. The carriage return is never injected, resulting in
stair-stepped early console output.

> +
> +		.macro	waituartcts, rd, rx
> +		.endm
> +
> +		.macro	waituarttxrdy, rd, rx

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260930-b4-cs75xx-v1-0-4617c4463529@gmail.com?part=5