Reducing RAM on AVR using PROGMEM

[espHorst]

Dear olikraus, thanks at first for this wonderful library! I would like to ask if it is possible to further reduce the RAM usage on AVR processors. On AVR

static const uint8_t u8x8_d_max7219_init_seq[] = {

goes to ram. If you agree I can try doing a PR including

#ifdef __AVR__
    #include <avr/pgmspace.h>
#endif

#ifdef __AVR__
static const uint8_t PROGMEM u8x8_d_max7219_init_seq[] = { 
#else
static const uint8_t sbox[256] = {
#endif

to put static const to PROGMEM. Do you think that there are any issues? Best regards, espHorst

[olikraus]

Well, yes, I know, this is a missing thing... However, there are several things to consider.

• pgmspace.h is already included: https://github.com/olikraus/u8g2/blob/master/csrc/u8x8.h#L116
• There are already several defines, which will help to use progmem area and which should be reused: https://github.com/olikraus/u8g2/blob/master/csrc/u8x8.h#L152-L185
• We need to update https://github.com/olikraus/u8g2/blob/master/csrc/u8x8_cad.c#L159 and use the progmem access macro
• We need to modify ALL sequences, which are passed to u8x8_cad_SendSequence. If I use fgrep -c u8x8_cad_SendSequence *.c | fgrep -v ":0" | cut -d ":" -f 2 | xargs | tr ' ' + | bc it will give me 451 calls. This means we need to reliable update 451 arrays.
• I am not sure whether there are some indirect calls to u8x8_cad_SendSequence(). So this is a risk.
• Ah, yes, one more thing: We need to check whether the linker garbage collector will still work. Like for the font arrays, it might be required to put each sequence into its own section.

All in all a lot of analysis and tests need to be done. This is why I hesitated to do this.

[espHorst]

Hm. What about adding an additional dedicated u8x8_cad_SendSequenceP() with P for progmem. Then piece by piece memory critical sequences can be changed. I would try to start with u8x8_d_max7219_init_seq ;-)

[olikraus]

The goal is to keep the code portable for all platforms. This means, u8x8_d_max7219_init_seq should be usable on AVR but also on ARM and ESP. This means, adding a function for u8x8_d_max7219_init_seq only will not help. We need to do this for all 451 arrays. I mean, I need a solution, which will work for all 451 arrays on all plattforms.

You can definitly do a local solution for your own project for one specific array. But then this is not something what I will takeover here.

[espHorst]

Hi olikraus, thanks for the feedback - I will do a local solution for just this project and use a bigger uC (STM32F1) for the next project! Thanks a lot!

[olikraus]

Sure, I am sorry for this, but this task indeed is a bigger topic.