Memory mapped flash access breaks Broadcom NAND controller based access

[Rafał Miłecki]

Hi,

Broadcom Northstar platform has SPI controller and NAND controller
v6.1. Devices can come with serial or NAND flashes (or both).

To provide early access to configuration data (NVRAM) stored on flash
Northstar provides MMIO mappings:
#define SI_NS_NANDFLASH            0x1c000000    /* NorthStar NAND flash base */
#define SI_NS_NORFLASH            0x1e000000    /* NorthStar NOR flash base */

It's possible to ioremap those and read flash content. Details:
Documentation/devicetree/bindings/nvmem/brcm,nvram.yaml
drivers/nvmem/brcm_nvram.c

The problem is that reading flash that way breaks NAND controller
based access using brcmnand driver. A simple readb() triggers errors
like:
[  120.098439] SQUASHFS error: xz decompression failed, data probably corrupt
[  120.105371] SQUASHFS error: Failed to read block 0x3b8c02: -5
[  120.111158] SQUASHFS error: Unable to read metadata cache entry [3b8c00]
[  120.117877] SQUASHFS error: Unable to read inode 0x6760785
or
[    3.120527] ubi0 error: 0xc04f0b3c: PEB 0 contains corrupted VID
header, and the data does not contain all 0xFF
[    3.130664] ubi0 error: 0xc04f0b4c: this may be a non-UBI PEB or a
severe VID header corruption which requires manual inspection

I assume that whatever hardware provides MMIO based access it
interferences with NAND controller.

Is that something that could be solved in brcmnand? Or should Linux
just don't use MMIO post-booting (after brcmnand initializes)? Is
there some generic cross-subsystem method to achieve that?

-- 
Rafał