AVR uses Hardward architecture with separate address space for program
memory (flash) and data memory (RAM). Normal program flow can only
access data memory which means that all variables - including const
variables - have to be copied into RAM to be accessible. (This happens
automatically during startup.)
It is possible to work around this limitation in software but that
can have severe impact on performance and/or API complexity. It is hardly
feasible to change NuttX interfaces in a way that would allow to make use
of this workaround.
On newer AVR families, there is an alternative option enabled by this patch.
These chips map part of their program memory (a 32kB window) into data
memory address space. This patch leverages this feature and adds support
for placing const variables into the mapped window. No copy to RAM is done
for them.
Const variables are therefore loaded directly from flash (not consuming
RAM) while still being available to be used by any NuttX interface.
Linker script of breadxavr board is changed to make use of these changes.
Tested by verifying string addresses - parameters in printf call
in a custom application (and also by running the application and verifying
its output.)
Documentation tested by build.
Signed-off-by: Kerogit <kr.git@kerogit.eu>
Some code paths in drivers/serial/serial.c load head and tail values
of receive and transmit circular buffers with interrupts enabled,
making it possible that the interrupt handler changes the value.
As noted in the code, this is safe as long as the load itself is atomic.
That is not true for 8bit architectures which fetch the 16-bit values
using two load instructions. If interrupt handler runs between those
two instructions and changes the value, the read returns corrupted data.
This patch introduces CONFIG_ARCH_LDST_16BIT_NOT_ATOMIC configuration
option which is automatically selected for AVR architecture. Based
on this option, head and tail values are reduced to 8-bit length
so the read remains atomic.
Patch was tested by building on rv-virt:nsh - disassembly of functions
from serial.c showed no difference which is correct as Risc-V does
not need to protect reads of these values. There should be no impact
for architectures that do not set the new configuration option.
It was also tested by by custom echo application running on AVR128DA28.
Signed-off-by: Kerogit <kr.git@kerogit.eu>
Initial support adds chip specific files like Kconfig, flags for the compiler,
and basic functionality like system timer. Some files are stubs derived
from corresponding ATmega files but without the functionality they are
supposed to add (to be done in later patches)
Part of the work is altered copies of corresponding files for ATMega MCUs.
Initially supported is AVR128DA28.
