Chopstx new branch STABLE-BRANCH-2 and version 2.4.

[NIIBE Yutaka]

Hello,

I forgot to send an announcement, so, here it goes.

In July, Chopstx 2.4 was released.

	tag release/2.4
	Tagger: NIIBE Yutaka <gniibe at fsij.org>
	Date:   Mon Jul 18 11:10:01 2022 +0900
	commit 2693fd9ec255db9690abd5ad212e2a269be82521

I created the branch STABLE-BRANCH-2 for 2.x series.

Major change in 2.4 is: I decided to use picolibc in rules.mk, because
all we need is string functions.  You can still use newlib by modifying
rules.mk.

I did a mistake for STABLE-BRANCH-2, when I developed using multiple
machines.  I wrongly pushed non-ff changes forcibly, forgetting I pushed
changes already.  If you follow STABLE-BRANCH-2 branch, please do:

	$ git reset --hard 2693fd9
	$ git pull

This means that I don't include changes in master to STABLE-BRANCH-2.


Because I had (possibly, wrong) impression of multi-core MCU will be
more common, I started SMP support in master.  It will be 3.x series.
Now, it somehow works for GNU/Linux emulation.

AFAIK: Sony Spresense has six cores, RP2040 is dual-core, and some
versions of ESP32 are also dual-core.  (For 64-bit, Kendryte K210
and K510 are dual-core.)

It was my misunderstanding that there is (will be) ESP32 with dual-core
RISC-V.


			*	*	*

My original intention was use of Chopstx 2 for Gnuk 2, with RISC-V MCU.

However, while the MCU, GD32VF103, is good for general use, I don't
think it's good for the particular use case of Gnuk Token, since reading
out the flash ROM may be more easier.  So, my initial plan was canceled.

In this summer, I evaluated CH32V103 from WCH, another RISC-V MCU, which
is a bit slower than GD32VF103.  Unfortunately, its "ISP" protocol is
not good for the use case of Gnuk Token, either, even if the situation
is better than GD32VF103.  In its ISP protocol, by the verify command,
the chunk size is 8-byte long.  Since guessing 8-byte data for all
possibilities takes long time, it's considered safe for general use.
For Gnuk, it were good if the chunk size was 64-byte long or longer.
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