[EV3 - help wanted]: Implement SPI flash block device driver

[laurensvalk]

We'll need to implement an SPI flash driver for EV3 so we can load programs on boot and save them during shutdown.

This is very similar to the existing implementation for SPIKE Prime., just with a different set of SPI commands.

We also already have SPI1 working with DMA for the display. Flash uses SPI0 with CS0. SPI0 is also used with CS3 for the ADC, for which we already have code as well. But since we only use flash on boot and during shutdown, we don't need to switch between them at runtime. This makes it relatively easy since it is consistent with existing Pybricks hubs, but the original EV3 firmware did this too.

The necessary Flash SPI commands and parameters can be found in the original firmware and u-boot or just from the data sheet, so everything is out there in principle.

[lilstevie]

Subscribing and keeping an eye on this to give some assistance when I have time and if it’s still required

[maehw]

Where does the firmware run from? Does it also reside on the SPI flash or is it run from some internal flash or other non-volatile external memory? XiP or copied to RAM before execution?

Are there EV3s out with different SPI flash models or only a single chip?

Do you think of this as

1. copy from SPI flash to RAM on boot
2. operate on RAM
3. write back to SPI flash on shutdown?

[dlech]

copied to RAM before execution?

This one.

Do you think of this as

1. copy from SPI flash to RAM on boot

2. operate on RAM

3. write back to SPI flash on shutdown?

Yup. Ideally we would only write things that changed for fastest possible shutdown.

[laurensvalk]

Hi @maehw ! Yes, the logic you describe is exactly right. But that logic is shared across all the platform and already exists, here. We do this on SPIKE Prime and Technic Hub as well.

So what we'd need is the equivalent of this but called block_device_ev3, essentially implementing:

• pbdrv_block_device_init
• pbdrv_block_device_store
• pbdrv_block_device_read

If this is something you're interested in, I could set up the boiler plate stuff, config, and API hooks for EV3 so it would be matter of "filling in" with the appropriate SPI setup and commands.

[maehw]

I am interested, but I'd prefer others to dive in deeper here as I am still lacking the big picture (and free time).

I guess the TI AM1808 does not have any internal flash memory, right?

So both firmware and programs need to reside in SPI flash storage (16 Mbyte)... or an external SD card. As I guess the SD card is optional, everything shall reside in the external SPI flash.

I also haven't every worked with protothread, but I guess that's no big deal.

[laurensvalk]

So both firmware and programs need to reside in SPI flash storage (16 Mbyte)... or an external SD card. As I guess the SD card is optional, everything shall reside in the external SPI flash.

Yeah. The firmware is < 512 Kb, so we have lots of space to spare. We're starting out pretty simple with no file system at all.

Loading the firmware from SPI flash is done by the u-boot bootloader, so we don't need to worry about it. We just need to implement SPI operations to read and write a given region elsewhere in flash.

I think I'll set up some of the boiler plate code next week. From there, it should be a matter of digging into the datasheet to determine what we need to set up, send, and receive.

[ArcaneNibble]

I am very interested in working on this or any other Pybricks effort if someone is willing to send me hardware. I'm an experienced embedded engineer looking for interesting projects, and I can get started right away. For a more "résumé" background, I've done work such as https://github.com/ArcaneNibble/wch-uf2 and https://github.com/ArcaneNibble/ArcaneNibble/blob/main/final-header.jpg , and, for a more "personal" background, I got a very early start in "STEM" thanks to the RIS with the yellow RCX brick.

Let me know if you want to chat more extensively outside of a GitHub issue (I'm currently not set up to interact on the Fediverse due to the ongoing issues regarding Fosstodon)

[laurensvalk]

Thanks @ArcaneNibble for your message! You do have a lot of interesting projects. It may be a while before we figure out how to best do all this but we're really happy about the renewed interest.

[faehn]

The necessary Flash SPI commands and parameters can be found in the original firmware and u-boot or just from the data sheet, so everything is out there in principle.

Where can we find these references? I apologize, I do not see what the flash part is or what datasheet, or the original firmware or u-boot.

Flash uses SPI0 with CS0. SPI0 is also used with CS3 for the ADC, for which we already have code as well. But since we only use flash on boot and during shutdown, we don't need to switch between them at runtime.

Taking a quick look, I see pbdrv_adc_init() has the chip select, then pbdrv_block_device_init() is called. Assuming that too would set the chip select. Is it my understanding adc would select its CS3 but then flash would select its CS0?

Perhaps I will wait for the boilerplate code you mention to clear up questions.