Make font cache optional

I realized that the lvgl font converter not only produces const structures, but also modifiable ones. Specifically lv_font_fmt_txt_dsc_t and lv_font_t structures. Wanting to save some RAM, I went ahead and made them all const. Running the code like that led me to a segmentation fault, as in get_glyph_dsc_id some of the font_dsc fields are modified, due to a caching logic.

I’d like to suggest the following:

  1. Hide this caching logic behind a compile switch (e.g. #if (LV_FONT_CACHE != 0))
  2. Make lv_font_t and lv_font_fmt_txt_dsc_t structures const
  3. If the font cache is enabled, add a cache pointer to lv_font_fmt_txt_dsc_t structure, that points to the separate non-const cache structure


I think if we added config options to disable font cache we needed to add a lot of other config options too where a few bytes could be saved.

It sounds good but doesn’t require any new config option. E.g. a non-const lv_font_fmt_txt_cache_t variable can be added to the font file automatically by the font converter.
This way both lv_font_t and lv_font_fmt_txt_dsc_t could be const. It saves 50-60 bytes / font so for 2-3 fonts it’s alrady a few hundred bytes.

This option is good enough for me, maybe with an option to make the cache pointer NULL.

I’ve opened an issue to add this to the generated fonts: