Documentation for using the GAS in ACPI tables to report debug UART addresses at
https://learn.microsoft.com/en-us/windows-hardware/drivers/bringup/acpi-debug-port-table
states the following:
- The Register Bit Width field contains the register stride and must be a
power of 2 that is at least as large as the access size. On 32-bit
platforms this value cannot exceed 32. On 64-bit platforms this value
cannot exceed 64.
- The Access Size field is used to determine whether byte, WORD, DWORD, or
QWORD accesses are to be used. QWORD accesses are only valid on 64-bit
architectures.
Documentation for the ARM PL011 at
https://developer.arm.com/documentation/ddi0183/latest/
states that the registers are:
- spaced 4 bytes apart (see Table 3-2), so register stride must be 32.
- 16 bits in size in some cases (see individual registers), so access
size must be at least 2.
Linux doesn't seem to care about this error in the table, but it does
affect at least the NOVA microhypervisor.
In theory we therefore have a choice between reporting the access
size as 2 (16 bit accesses) or 3 (32-bit accesses). In practice,
Linux does not correctly handle the case where the table reports the
access size as 2: as of kernel commit 750b95887e5678, the code in
acpi_parse_spcr() tries to tell the serial driver to use 16 bit
accesses by passing "mmio16" in the option string, but the PL011
driver code in pl011_console_match() only recognizes "mmio" or
"mmio32". The result is that unless the user has enabled 'earlycon'
there is no console output from the guest kernel.
We therefore choose to report the access size as 32 bits; this works
for NOVA and also for Linux. It is also what the UEFI firmware on a
Raspberry Pi 4 reports, so we're in line with existing real-world
practice.
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1938
Signed-off-by: Udo Steinberg <udo@hypervisor.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: minor commit message tweaks; use 32 bit accesses]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
41f7b58b63