New year, new blog

At the beginning of July last year, I started to create my devblog. At that moment, everything web-related was completely out of my world. I could code a bootloader or a real-time OS in C/C++, yet I wouldn't be able to make even a basic website. So I took the easy way. I registered my domain, bought three years of the cheapest VPS provider I could find, and installed a ready-made blog solution, the ubiquous Wordpress. The only real customization I did, was to write a theme - or should I say, download an existing one, and rewriting the CSS part.

It was running well, until it did not anymore. I'm new to this, so I didn't know that it is a terrible idea to buy three years worth of VPS service to a company that nobody knows. It turns out the provider, Hiformance, went bankrupt (or just ran away with the money), shutting down all the servers without notice.

So my blog was gone. Of course, I didn't have any backup. I could thanksfully recover most of it with the help of my browser cache, and the Wayback Machine. I got back the blog posts, comments, images, and CSS stylesheet. I don't like doing the same thing twice, though, so I decided not to buy another VPS service just to run a Wordpress instance. This time, I would write my website from zero, and learn in the process.

One of the less-known features of Github / Gitlab, is that these code hosting services can host for free a simple static website. It's as simple as pushing your HTML and assets to a branch named 'gh-pages' (or simply 'pages' on Gitlab). Then, your website appears at Very useful for hosting simple web pages for your projects. It is, however, much harder to have it host complex websites, such as a blog, for the simple reason that you cannot run a server language like PHP or a database on their servers. You're just left with HTML and Javascript.

Enters Jekyll

In a nutshell, Jekyll can be described as a static website generator. Write up rules about how your pages should look, give it text written in Markdown or plain HTML, and it will generate a static website from that. New blog post? Add the text of the post to a Markdown or HTML file, regenerate the website, push the generated HTML files to Github, and your new blog post is live. Actually, it's even simpler than that: Github's simple hosting service supports Jekyll natively, so just push your sources to the gh-pages branch instead of the generated HTML, and Github will auto-generate the static website from your Jekyll sources. How cool is that?

One last part that's left to do: comments. The first problem: this requires a server-side program to get the data submitted by the user through the HTML form. The second problem: How do you get this data integrated into your website, if your website is a set of static pages generated from a git repository?

Enters Staticman

Staticman is a tool that has been designed specially for the purpose of handling comments in generated static websites. The way it works: you setup your blog post reply form to send the comments to the public instance of Staticman that they graciously offer free of charge (alternatively, you can also run your own instance on your server). Second step, you add 'staticmanapp' as a collaborator to your website git repository. Then, everytime someone posts a comment, 'staticmanapp' will send you a pull request, adding the new comment in a file stored in a preconfigured directory. Merge it, let Github automatically regenerate the website, and your new comment will appear. Pretty nice, right?

Bottom line

So it took a bit of effort, but now my devblog is live again. It looks the same, but has been entirely written from scratch this time, not by just tweaking a Wordpress theme. And you know what? I'm glad I did. I learned a lot in the process. Also, I really got used to write my blog posts the same way I write my code. Fire up VIM, write some text, git-add, git-commit, git-push. Tweak the look-and-feel and content by writing code, instead of clicking buttons.

The best thing about that? You can now send me guests posts in pull requests ;)

Saving my Pokémons

The last few days I’ve been toying with the possibility of installing OpenDingux directly on the internal NAND. What I’ve been doing so far, was to boot the kernel from USB, and have it load the root filesystem (an unmodified OpenDingux rootfs for the Dingoo A320) from the SD card. It worked fine, but I had to unplug the SD card everytime I wanted to boot the kernel (required for booting over USB), and I didn’t like the idea of having proprietary software on the NAND.

Problem was, to be able to install OpenDingux on the NAND, I had to format the NAND first. And lose my savestate of Pokémon Emerald, on which I spent more than 30 hours.

After figuring out the changes needed to the memory controller and the NAND controller, I was able to make a full dump of the NAND directly to the SD card, complete with Out-Of-Band (OOB) data (that’s the part that contains meta-data, like error correction codes). I then got the BCH (error correction) controller to work, so I could make a dump of the NAND without the OOB.

The problem is that a dump of the NAND is useless if you don’t know how the data is stored.

NAND flashes are very prone to wear. Each eraseblock (128 KiB of data) can be written only about 1000 times before it starts to fail. To counter this issue, the OS has something called a Flash Translation Layer (FTL). It is basically a table that maps logical eraseblocks to physical eraseblocks. With the FTL, if you store a MP3 on the NAND, your OS will see the file as a continuous stream of bytes till the end of the file, but in reality each block of 128 KiB will be stored at a pseudo-random address somewhere in the whole NAND. If you overwrite the MP3 with another one, the file’s data will appear at the same address, but the new data written will be stored in completely different eraseblocks. The FTL is then a mechanism to prevent the damage done to the eraseblocks by repeated writting to the same files.

Back to my problem. By hex-editing the NAND dump I figured that the filesystem used was FAT16; the problem was that the filesystem lies on top of a FTL, so it was cut in 128 KiB pieces scattered across the whole NAND. I had to figure out a way to map each physical eraseblock to a logical eraseblock. Sure enough, by spending some time hex-editing, I figured out that the first page of each eraseblock starting from address 0x20000 had incorrect error-correction data in its OOB area. By looking closer, I could figure out that one 32-bit word contained a different number for each erase block: most likely that value corresponded to the logical erase block that was mapped to that physical erase block. And sure enough, after modifying the FTL driver that was used on the Dingoo A320 to fit the Retromini’s NAND, I could access the files and save my precious Pokémons!

Now, to finish this post, I have a good news and a bad news.

I’ll start with the good one: I cleaned and pushed my jz-4.18-retromini branch on Github. It contains all the work I did so far on the Retromini, including the FTL driver. By combining all the information I’ve written in this blog so far, it should be possible for other people to boot their own kernels.

The bad news: I know that will deceive some, but I think I’m done with this for this year. I’m about to start a 1800km walk, then probably travel in Spain and South America for a bit. My Retromini will stay at home, like my laptop.

Till next time!

Linux on the Retromini

Time for a progress report.

I successfully booted a Linux kernel on the Retromini some weeks ago. I won’t paste the whole kernel boot here, but you can read it by clicking here.

Since then, I"ve been adding support for the various hardware components of the Retromini in the upstream drivers if they exist, or writting new drivers otherwise.

Here is a list of the hardware components, and their current state:

UART Working Upstream
IRQ controller Working Upstream
PLL and Clocks Working Not upstream
Clocksource Working Sent upstream
System timer Working Sent upstream
Watchdog (reboot) Working Sent upstream
PWM (backlight) Working Sent upstream
RTC (power-off) Working Upstream
Pinctrl + GPIO Working Sent upstream
DMA Working Sent upstream
SD card Working Upstream
Memory controller Working Not Upstream
NAND Working Not Upstream
BCH (error correction for NAND) Working Not Upstream
Audio Not working
USB Ethernet / mass storage Not working
Buttons Working except SELECT Upstream
ADC (SELECT button) Not working
LCD Not working

If I find some time in the next few days, I will clean up and push on Github the current git tree I’m working on.

Some things I thought I’d share:

  • The JZ4725B SoC used by the Retromini is actually newer than the JZ4740. Some hardware blocks are more similar to the JZ4770 than the JZ4740.
  • The RTC does not retain the date across reboots. This sucks, as I won’t be able to play Pokémon games then, as they rely a lot on time. Maybe a hardware modification could make it work?
  • The buttons are for the most part GPIO-mapped. The R button is not available when serial logging is enabled, is it uses the same pin as RX. Also, for some reason the SELECT button is not GPIO-mapped, but connected to the Analog-to-Digital Converter (ADC). Whoever thought it was a good idea, should burn in hell.

That’s all folks!

UBIBoot on the RetroMini

Hello there,

For those who don’t know, I had written a bootloader called UBIBoot, quite a long time ago, for the Dingoo A320. As its name suggests, it supported booting from a UBI volume on the NAND. That is to say, it was able to boot a Linux kernel on the only Dingoo A320 with the internal NAND formatted as UBI: my own. I trashed the native OS and was running OpenDingux directly on the internal NAND.

The job of UBIBoot was to initialize the main PLL, the clocks, set the pins of the SoC in the right mode, initialize the SDRAM, initialize the NAND, initialize the SD card controller, and then finally, boot Linux from the first FAT partition of the SD card if there’s one inserted, or from the “kernel” UBI drive of the internal NAND.

All of that in about 5 kilobytes.

Before the GCW Zero came out, I got a prototype (with 256 MiB RAM), and ported UBIBoot to it. It then became the default and official bootloader for the GCW Zero when it was retailed months later.

There’s a fun story about it: the GCW Zero prototype didn’t have marks on the main board to show where the UART pads were, so I ported UBIBoot without having any output log: instead, I was blinking the LCD. My test passed? LCD turned on. It failed? The LCD remained off. Repeat for everything you need to test in your code. Next time it will be faster for me to learn Morse code and make the LCD blink Morse code to me 🙂

Anyway, I disgress. I ported UBIBoot to the RetroMini, and the code is in the master branch now, which means it’s officially supported. The code is available here.

Note that right now, it only supports booting from an external micro-SD card. I did not try to flash it on the NAND; I want to dump the NAND before starting to overwrite it with random stuff.

To compile:
CONFIG=rs90 make

For the stage1 variant, where UBIBoot will only initialize the hardware then return to the USB loader:
CONFIG=rs90 STAGE1_ONLY=y make

Note that right now, it’s only useful for people with a UART cable soldered and who want to boot their own Linux kernel (or other operating system).

Next step: boot Linux!