XMC Flash chip output drive support

[ChocolateFrogsNuts]

Basic Infos

• [ X] This issue complies with the issue POLICY doc.
• [X ] I have read the documentation at readthedocs and the issue is not addressed there.
• [X ] I have tested that the issue is present in current master branch (aka latest git).
• [X ] I have searched the issue tracker for a similar issue.
• [X ] If there is a stack dump, I have decoded it.
• [X ] I have filled out all fields below.

Platform

• Hardware: other - XMC Flash chips attached to any ESP8266
• Core Version: [-]
• Development Env: [Arduino IDE]
• Operating System: [N/A]

Settings in IDE

• Module: [Wemos D1 mini]
• Flash Mode: [qio]
• Flash Size: [4MB/1MB]
• lwip Variant: [v2 Lower Memory]
• Reset Method: [ck|nodemcu]
• Flash Frequency: [40Mhz]
• CPU Frequency: [80Mhz|160MHz]
• Upload Using: [SERIAL]
• Upload Speed: [460800] (serial upload only)

Problem Description

XMC Flash chips used on a number of boards (most notably some WEMOS D1 mini) have a power saving feature that defaults to 75% drive on their outputs. This can result in unstable operation at 40MHz and above in some circumstances.

While #6552 allows lowering the flash frequency to achieve stable operation, it is possible to run the chips at the full 80MHz with 100% drive by setting bits 5 and 6 of SR3 with
esptool.py write_flash_status --bytes 3 0x600000
Unfortunately this only lasts for the current power cycle.

Following on from the discussion started at the end of #6366, we now attempt to develop some code that will set SR3:5,6 in XMC flash chips when they are detected...

[ChocolateFrogsNuts]

Ok, got code that sets SR3 at https://github.com/ChocolateFrogsNuts/ESP-DebugTools/blob/master/DebugTools/flash.ino
but for some reason it's not able to read SR3 yet... once that's sorted I can organize a PR with some neater/stripped down code suitable for a startup check.

[ChocolateFrogsNuts]

Update: got it reading the flash status registers properly.
We now have a working function for sending generic SPI commands, and a short function that uses it to enable 100% output drive if an XMC flash chip is detected at >26 MHz.

I'll work on a PR over the next few days. 392 bytes of the code need to be IRAM_ATTR (although I can get that down to 270 by making certain assumptions), so this probably needs to be something that can be enabled/disabled easily.

[earlephilhower]

I wonder if we can do some tricks for functions that are only needed a single time to fix them in the ICACHE and not permanently in IRAM.

Could we disable all interrupts, then read the function code into a dummy variable, to cause it to load into IRAM? Then call the fcn, which since no no-IRQ code could have caused it to spill out of the cache. Some question as to the # of ways the cache uses, etc. But seems better than wasting such a large chunk for a one-time event.

Alternatively, overlays could be used and you could explicitly flush out existing IRAM, write your fcn in, call it, and then write the old code back. It's hackish, but if its done at chip init time (preInit callback, see the examples) it may be very low risk.

[devyte]

@earlephilhower whatever solution works regarding those ideas, it would be worthwhile to search other cases to apply it to.
I particularly like the idea of manually preloading a function into cache. To not make it so hackish, an api could be implemented to ease usage.

[ChocolateFrogsNuts]

I've been looking at the compile stats we get now and thinking the same things - there seems to be a lot of ICACHE_RAM_ATTR code, and surely a lot of it only runs rarely or even once.

One thing I have noticed is that functions seem to be cached a page at a time as needed.
If there was a way to force pre-caching an entire block of code or function while it was executing, then release it... perhaps with overlays or by placing each function in it's own linker section.
If we could have linker sections loaded into IRAM/released on demand it would be perfect.
In this case we don't need speed, we just can't allow a page fault during the "critical" part.

[mhightower83]

@ChocolateFrogsNuts, How big is a page? Assuming the whole function fits into a single page, it sounds like it would be loaded before execution, is there a problem? If it's more than one page, what if you break it into parts. Such that you can call the parts for preloading, then call them for real. Something like this:

void part1(int something, bool preload) {
  if (preload)
    return;
 // ...
  part2(something, false);
}
void part2(int something, bool preload) {
  if (preload)
    return;
 // ...
}
part1(0,true);
part2(0,true);
part1(42,false);

[ChocolateFrogsNuts]

I am assuming flashchip->page_size (256 bytes), or a multiple of it. It's not very big.
If spi0_command() is not IRAM_ATTR, or if there is a call to a non-iram func in it (eg Serial.print) it triggers a page fault, invalidating the SPI registers that have already been set - usually resulting in a WDT reset.
The problem is it needs to get all the way from setting the first register to copying the result without anything else accessing the SPI controller. I don't know if SPI1 can be successfully used rather than SPI0 to access the flash chip, but that would still have a concurrent access problem...

Hmm, I wonder if I could reduce the critical size enough by calculating the register values in one func, then just loading them into the controller and reading the result in a separate func...

EDIT: the func would need to be page-aligned to give that a chance of working

[ChocolateFrogsNuts]

Split the func in 2, getting the critical section down to 160 bytes, but it still has to be IRAM_ATTR.

Anyone know how to align the func? "#pragma align(256)" is ignored because the compiler doesn't understand it (a warning is printed).

EDIT: __attribute__((aligned(256))) seems to compile, but still fails without IRAM_ATTR.

[ChocolateFrogsNuts]

I have a suspicion the page size is 64 bytes - the maximum size of a transfer by the SPI hardware controllers.

[ChocolateFrogsNuts]

For future reference, executing code in RAM causes Exception 2 - InstructionFetchError
so we can't copy a func into normal RAM while it runs.
Also xtensa supports an IPFL (Instruction PreFetch and Lock) to temporarily lock code into the cache which would have been perfect... but the ESP8266 does not have that option enabled :(
The cache page (line) size also appears to be 16 bytes, so that pretty much rules out any attempt at preloading.

[earlephilhower]

@ChocolateFrogsNuts how about (over)writing 32b wide into IRAM and executing that? That's less appealing, since you need to copy back the real code when done, but if done in the preinit (see WiFiShutdown.ino) should be relatively safe.

[ChocolateFrogsNuts]

So far I get exception 3 - LoadStoreError just trying to read the iram.

[devyte]

What happens if you run this configuration code as part of eboot? I believe that any IRAM used gets cleared when booting the application.

[ChocolateFrogsNuts]

ahh, oops - my bad...
I tried iram access by doing a memory dump... inadvertently accessing it as uint8 - which was never going to work as iram can only be accessed as aligned uint32
I will see what I can do with that....
I'll take a look at doing it in eboot if this doesn't work out.