serial: 1 KB receive buffers on F7 and H7

[MrScothh]

What this fixes

A receive buffer has to hold what arrives between two runs of the task that drains it, which
for the GPS is every 20 ms. At 115200 that is 230 bytes and 256 is enough, which is why the
current size has served; at 230400 it is 460 bytes and the buffer overruns on every burst.

What that costs is not a few bytes here and there. On an H743 bench with a NEO-F10N at
230400, INAV never identified the receiver at all: packets kept arriving, more than 2800 of
them in 80 seconds, while the hardware version stayed unknown and the timeout counter climbed
to 26. UBX-MON-VER, the reply that carries it, takes 11 ms of line at that rate, so an
overrun always lands in the middle of it and the answer is lost, every five seconds, forever.
With 1 KB buffers the same receiver was identified immediately, with no timeouts and no
errors.

What I changed

The receive buffers are 1 KB on F7 and H7 and stay at 256 bytes everywhere else.

Family	Ports a target defines	Cost	Free RAM in the maintenance build
H7	up to 8	6 KB	363 KB on the smallest target
F722	up to 6	4.5 KB	96 KB
F745 and F765	up to 8	6 KB	137 KB
F405	up to 6	-	13.7 KB, so left alone
AT32F435	up to 8	-	1.5 KB, so left alone

The note about occupied sizes being returned as uint8_t no longer held: the driver
functions and the ring indices are all 32 bit.

Testing

On a TBS Lucid H7 Wing the change adds 5376 bytes of RAM, 768 for each of the seven ports the
target defines, and the receiver at 230400 is identified in about 2 s rather than not at all.
Built for MATEKF722SE, where RAM goes from 45.7% to 47.7%, and for MATEKF405SE, which is
unchanged.

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PR Summary by Qodo

Increase F7/H7 UART receive buffers to 1 KB

🐞 Bug fix ✨ Enhancement 🕐 10-20 Minutes

AI Description

• Expands F7 and H7 UART receive buffers to prevent high-rate GPS data loss.
• Retains 256-byte buffers on memory-constrained MCU families.
• Corrects outdated documentation about serial buffer size limitations.

Diagram

graph TD
  FAMILY{"MCU family"} -->|"F7/H7"| LARGE["1 KB default"] --> UART["UART1-8 RX"] --> RING["Driver buffers"] --> GPS["GPS parser"]
  FAMILY -->|"Other MCU"| SMALL["256 B default"] --> UART

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High-Level Assessment

The following are alternative approaches to this PR:

1. Per-port configurable buffer sizes

• ➕ Limits additional RAM to ports handling bursty high-rate devices.
• ➕ Allows targets to tune buffers for their specific UART assignments.
• ➖ Requires target-level configuration and maintenance across many boards.
• ➖ UART roles can be reassigned at runtime, making safe defaults harder to guarantee.

2. Drain UARTs more frequently

• ➕ Could reduce backlog without increasing static RAM usage.
• ➖ Adds scheduler load and timing risk across targets.
• ➖ Does not guarantee that a 258-byte reply survives competing navigation traffic.

Recommendation: Keep the family-wide compile-time default used by this PR. F7/H7 targets have sufficient RAM, and uniform 1 KB buffers protect dynamically assigned UART roles without target-specific configuration or scheduler changes.

Files changed (1) +22 / -11

Bug fix (1) +22 / -11

serial_uart.hSelect larger UART RX buffers for F7 and H7 +22/-11

Select larger UART RX buffers for F7 and H7

• Introduces a family-dependent default that assigns 1 KB receive buffers to every UART on STM32F7/H7 while retaining 256 bytes elsewhere. It also documents the high-rate GPS overflow scenario and removes the obsolete claim that occupied sizes are limited to uint8_t.

src/main/drivers/serial_uart.h

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