cpu/esp8266: most .rodata sections are moved from DRAM to IROM (flash)

[gschorcht]

Contribution description

With this PR most .rodata sections are moved from DRAM to IROM (flash) which makes more RAM available for applications.

Background

Since IROM (flash) access requires 32-bit word aligned reads, usually all .rodata sections are placed in DRAM by the Espressif SDK.

Solution

Using the LoadStoreError exception handler from esp-open-rtos makes it possible to place .rodata sections also in IROM (flash) to save RAM resources.

This PR adds the LoadStoreError handler and moves most of the .rodata sections to IROM, saving 12 kBytes of DRAM for an application such as examples/gnrc_networking. Since the LoadStoreError exception handler may reduce the performance of IROM access, performance-critical parts of .rodata section, e.g., core.a, cpu.a and periph.a are still kept in DRAM.

Note

This PR is only relevant for applications that use the SDK version of the RIOT port for ESP8266 where available memory is critical. In non-SDK version available memory is not critical.

Testing procedure

1. Compile examples/gnrc_networking with and without the PR and check the size of .rodata in the output of size command.

   USEMODULE=esp_wifi make BOARD=esp8266-esp-12x -C examples/gnrc_networking
   size -Ax examples/gnrc_networking/bin/esp8266-esp-12x/gnrc_networking.elf
   

   Without PR you should get something like:

   size -Ax examples/gnrc_networking/bin/esp8266-esp-12x/gnrc_networking.elf
   ...
   .rodata           0x3fb0   0x3ffe8fa0
   .bss              0xb0d0   0x3ffecf50
   ...
   

   That is .rodata has a size of 0x3fb0 (16.304 bytes) and is placed at 0x3ffe8fa0 which is in DRAM. With PR you should get something like:

   size -Ax examples/gnrc_networking/bin/esp8266-esp-12x/gnrc_networking.elf
   ...
   .rodata            0xfb0   0x3ffe8fa0
   .bss              0xb0e0   0x3ffe9f50
   ...
   

   .rodata is much smaller0xfb0 (4.016 bytes) and .bss starts at lower address. That is, much more memory is available for dynamic memory allocation which is essential for the ESP8266 WiFi interface.

The difference of about 12 kBytes is additionally available as RAM.

2. Flash the application and use command help

   USEMODULE=esp_wifi make BOARD=esp8266-esp-12x -C examples/gnrc_networking flash test
   > help
   

   Help text is read from IROM.

Issues/PRs references

This PR depends on #10980.
Changes of this PR have to be ported to PR #10883.

[gschorcht]

@smlng I also would like to get it merged before code freeze since it is an important enhancement which saves a lot of RAM.

[smlng]

if I understand correctly this is only moving memory around but does not safe bytes, right? Because in total I see an increase of 500 bytes compared to master, but I also see the mem shift as you described using size -Ax.

Master:

280909	   3989	  45264	 330162	  509b2	/data/riotbuild/riotbase/examples/gnrc_networking/bin/esp8266-esp-12x/gnrc_networking.elf

section             size         addr
.data              0xf95   0x3ffe8000
.rodata           0x3fe0   0x3ffe8fa0
.bss              0xb0d0   0x3ffecf80
.text             0x723a   0x40100000
.irom0.text      0x39733   0x40210000

This PR:

281417	   3989	  45280	 330686	  50bbe	/data/riotbuild/riotbase/examples/gnrc_networking/bin/esp8266-esp-12x/gnrc_networking.elf

section             size         addr
.data              0xf95   0x3ffe8000
.rodata            0xf7c   0x3ffe8fa0
.bss              0xb0e0   0x3ffe9f20
.text             0x73ea   0x40100000
.irom0.text      0x3c7e3   0x40210000

[gschorcht]

@smlng Yes, you are right, it does not save any bytes of code. But, code size isn't a problem since you have between 1 MByte and 4 Mbyte flash memory.

The problem is that .rodata section isn't placed in flash as usually expected but in RAM. The reason is that flash requires aligned 32-bit word access. Therefore, the goal of this PR is to move const data to flash memory .irom0.text section to save RAM. The difference of .rodata section size is the RAM you save which can be used as heap which is especially important for the WiFi interface.

[smlng]

tested ACK!

[gschorcht]

@sming Thanks a lot for reviewing and testing.