Feature Request: RP2350 partition support with XIP_NOCACHE_NOALLOC_NOTRANSLATE_BASE for cross-partition access

Problem Description

The current flash block device implementation uses XIP_BASE (0x10000000) for reads, which works perfectly for RP2040 and single-partition RP2350 setups. However, on RP2350 with partitioned flash, the bootrom restricts XIP_BASE to only map the currently booted partition. This prevents applications from accessing data stored in other partitions.

Technical Background

On RP2350, when using A/B firmware partitions with a separate data partition:

• Partition 0: Firmware A (0x002000-0x102000)
• Partition 1: Firmware B (0x102000-0x202000)
• Partition 2: Data (0x202000-0x602000)

The bootrom configures XIP_BASE to only map the active firmware partition. Attempting to read from the Data partition via XIP_BASE + data_partition_offset results in bus faults and system hangs.

Proposed Solution

Add support for XIP_NOCACHE_NOALLOC_NOTRANSLATE_BASE (0x1c000000) as an alternative read method for accessing the entire flash, regardless of partition restrictions.

Implementation Approach

1. Detection Logic: Add runtime detection for RP2350 and partitioned flash
2. Address Selection: Use XIP_NOCACHE_NOALLOC_NOTRANSLATE_BASE when accessing flash outside the booted partition
3. Backward Compatibility: Maintain existing behavior for RP2040 and non-partitioned RP2350

Use Cases

• Cross-partition file systems (e.g., LittleFS on dedicated data partition)
• Configuration storage in separate partition from firmware
• A/B firmware updates with persistent data storage
• Multi-partition applications requiring access to shared data

Alternative Workaround

We successfully implemented a custom block device that uses XIP_NOCACHE_NOALLOC_NOTRANSLATE_BASE for reads while maintaining compatibility with the existing pico-vfs API. This could serve as reference implementation for the upstream fix.

References

• RP2350 Datasheet Section 2.11.5.2 - XIP address translation
• Forum discussion - Engineer confirmation of XIP_NOCACHE_NOALLOC_NOTRANSLATE_BASE usage