Comment 3 for bug 1809546

Yes, our PL011 implementation assumes that you only ever access the 32-bit registers with full width 32-bit word reads and writes. Don't try to do byte accesses to them. The PL011 data sheet doesn't specifically say that partial-width accesses to registers are permitted, so I think that trying to access offset 0x39 falls under the general note in section 3.1 that attempting to access reserved or unused address locations can result in unpredictable behaviour.

You need to make sure you write your C code in a manner which enforces that accesses to device registers are done as single 32-bit accesses, and the compiler does not silently break them down into multiple reads and writes, or you will be in for a lot of pain trying to figure out what is going on if the compiler ever does it with registers that are write-to-clear or similar behaviour. Linux, for instance, does this by having readl() and writel() functions that end up doing inline asm of ldr/str instructions.