evaluate complexity of moving immutable device data to substructure

[pabigot]

As requested to support of assessing options related to constification of struct device (#26072) this PR demonstrates the conversions for and impact of placing immutable device data in a substructure, with only the mutable material (and a pointer to the substructure) in the RAM section which remains referenced as struct device * in all API (i.e. is not const qualified).

Those who are interested can use this branch to evaluate the savings on specific applications. I picked two bluetooth samples on two boards; it appears the cost is on the order of 2 to 3 bytes of flash for each byte of RAM saved, but I wouldn't infer too much from that.

I was mistaken in the 2020-06-10 dev-review when I surmised that the conversion would be difficult. Coccinelle handles about 90% of the required modifications to inject the fixed pointer dereference; another remaining 9% are handled with a sed script; and a few need to be done by hand. The conversion is significantly simpler than converting to const struct device * in #25208, though the sed script could make inappropriate changes, and this draft PR probably still hasn't identified all the places where a change needs to be made.

The conversion does affect over 540 files, but the impact on non-driver code is minimal. The steps in the PR are fragile in the face of upstream changes: mechanical rebases will generally fail so the last two or three commits must be implemented by re-running the transformations.

For samples/bluetooth/central_hr on pca10056: -56 ram +110 flash
   text	   data	    bss	    dec	    hex	filename
 145070	   2466	  20355	 167891	  28fd3	build/zephyr/zephyr.elf   # RAM
 145280	   2410	  20355	 168045	  2906d	build/zephyr/zephyr.elf   # Flash

For samples/bluetooth/st_ble_sensor on wb55: -88 ram +260 flash
   text	   data	    bss	    dec	    hex	filename
  62096	   1956	  16823	  80875	  13beb	build/zephyr/zephyr.elf   # RAM
  62356	   1868	  16823	  81047	  13c97	build/zephyr/zephyr.elf   # Flash

[pabigot]

When rebasing on master the easiest approach is to cherry the non-automated commits, then run each of these commands before committing the result with the corresponding application commit.

Use this as the change for "DNM device: update tests of init result"

spatch --sp-file initres.cocci \
  --very-quiet --macro-file scripts/coccinelle/macros.h \
  --include-headers --dir . \
  | sed -r \
    -e 's@res([!=]=)(\S)@res \1 @g' \
    -e 's@res([!=]=\s)@res \1@g' \
  > initres.patch

Use this as the change for "DNM device: fixups for fixed data into separate structure"

spatch --sp-file devfix.cocci \
  --very-quiet --macro-file scripts/coccinelle/macros.h \
  --include-headers --dir . \
  > devfix.patch

Use this as the change for "DNM device: convert helper macros that access immutable device fields"

FTAG="(name|config_info|driver_api|driver_data|device_pm_control|pm)"
git grep -lP '.->'"${FTAG}"'\b' \
  | sed \
    -e '/workqueue.rst/d' \
  | xargs sed -i -r \
    -e 's@(\(\w+\))(->'"${FTAG}"'\b)@\1->fixed\2@g' \
    -e 's@\b(_?dev)(->'"${FTAG}"'\b)@\1->fixed\2@g' \
    -e 's@\b(\w+_dev)(->'"${FTAG}"'\b)@\1->fixed\2@g' \
    -e 's@\b(\w*_?device)(->'"${FTAG}"'\b)@\1->fixed\2@g'
sed -i -r \
  -e 's@(ppp_dev|at_dev|control_dev)(->'"${FTAG}"'\b)@\1->fixed\2@g' \
  drivers/modem/gsm_ppp.c

[pabigot]

Outdated; this approach was rejected in favor of making struct device const.