Hello,I would like to submit patches which change handling of const variables in AVR architecture, DA/DB family.
AVR is a Harvard architecture with separate address spaces for program and data memory. Unless the compiler is instructed otherwise, C pointers are always referring to the data memory. Variables declared with const must therefore be copied to RAM to be accessible - which is undesirable since RAM is a limited resource on these chips. There are other approaches that can be taken but they are either not supported in NuttX or the support is quite limited. (These approaches are described in the document added in patch 0002 and adding full support for them would require fairly disruptive changes so that is unlikely to happen.)
Newer chips offer an alternative solution - part of the program memory (a 32kB window) is mapped into data memory address space. Const variables can be placed into this window, making them accessible while using standard (data memory) pointers. This patch series adds support for this into both AVR architecture code and Breadxavr board.
Series was tested by verifying string addresses - parameters in printf call in a custom application - in disassembly (and also by running the application and verifying its output.) Documentation was tested by HTML build.
Short summary of the patches: 0001 - fixes Ctrl-C error in a comment, unrelated otherwise0002 - adds a document that describes previously existing options of avoiding copying const variables into RAM
0003 - patch that makes use of the memory-mapped flash 0004 - ops structure in serial driver is marked as constThis patch series should not affect older AVR chips which do not have the memory-mapping.
All patches are attached to this message and also available in a git repository nuttx.git at git.kerogit.eu accessible through HTTP/S. (Trying to prevent bot traffic by not posting the URL in machine-readable form.) The relevant branch is called avrdx_flmap_v1. If the patches are acceptable, I would like to ask someone with GitHub account to open a PR (I don't have a GH account.) Any comments or suggestions are welcome.
0001-boards-avr-avrdx-breadxavr-fix-path-in-license-heade.patch.gz
Description: application/gzip
0002-Documentation-platforms-avr-document-options-of-keep.patch.gz
Description: application/gzip
0003-arch-avr-avrdx-do-not-copy-const-variables-into-RAM.patch.gz
Description: application/gzip
0004-arch-avr-avrdx-avrdx_serial-make-uart_ops_s-structur.patch.gz
Description: application/gzip
