Fix check for rtc mem sdk api in ESPClass

[devyte]

Also:
-added extra check to disallow zero-size read/writes
-added some comments to explain the rtc mem layout, api, interface
Fixes #2875

[devyte]

Looks like I messed something up with git, but I think it should be resolved once #5327 is merged.

[mcspr]

I think it should also mention Updater/eboot interaction, see #619 (comment)

Arduino/bootloaders/eboot/eboot_command.c

Line 48 in 74819a7

void eboot_command_write(struct eboot_command* cmd)

Arduino/cores/esp8266/Updater.cpp

Line 199 in 055748f

eboot_command_write(&ebcmd);

OTA update will overwrite first 128 bytes of user section

[devyte]

@mcspr I don't think that's correct. I think eboot is using the first 128 bytes (i.e.: first half) of the system data, not part of the user data.
What is interesting in your linked comment is that the second half of the system data appears to not be used at all. Per the referenced testing, it seems to be safe to use by the user. Still, it is specified as reserved by Espressif, which means it could be used in the future.

[mcspr]

Per https://github.com/esp8266/esp8266-wiki/wiki/Memory-Map#memmory-mapped-io-registers
0x60001100 is system section, for 0x100 / 256 bytes
0x60001200 is user section

Arduino/bootloaders/eboot/eboot_command.h

Line 15 in 055748f

#define RTC_MEM ((volatile uint32_t*)0x60001200)

eboot writes to the user portion here, am i wrong?
e.g. writing using 64 offset and reading directly from that address would return the same data value:

#define RTC_MEM 0x60001200
uint32_t data = 1751720192;
system_rtc_mem_write(64, &data, 4);
data = ((uint32_t*)RTC_MEM)[0];
Serial.println(data);

(nodemcu source used as reference, checking how rtcmem used there)

edit: and sorry for hijacking the pr, as it is only partially related. maybe this is better served as a documentation issue instead of adding code comments.

[devyte]

@mcspr I misread the RTC_MEM define at a glance.
So you're right: that means that the first 128 bytes of the RTC user mem can't be used if Updater is in play. Please open an issue with request for documentation, and reference this discussion. In the meantime, I'll update the comments in this PR.

Note: comments should be added to the Updater section and ESP-specific APIs section.

[devyte]

Closing in favor of #5372 .