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add a proposal on how to handle risc-v images

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Pratham Patel 2023-04-29 16:42:52 +05:30
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# Disk image partition layout for the VisionFive 2 SBC
The VisionFive 2 SBC has two dip switches on board which dictate how the board boots. Two switches mean 4 combinations and by extension, 4 modes in which this SBC can boot. These modes are as follows:
| **Boot mode** | **RGPIO_0** | **RGPIO_1** |
|--------------------------|-------------|-------------|
| QSPI flash (on-board) | 0 | 0 |
| SD Card | 1 | 0 |
| EMMC (add-on) | 0 | 1 |
| UART (firmware recovery) | 1 | 1 |
**NOTE: I don't have an EMMC module so I haven't tested the 3rd option of EMMC boot. But I changed the dip switches to boot from EMMC and it refused to boot because the EMMC flash module was not detected. Based on that, I can conclude that SD Card boot and EMMC boot behave the same way, the only difference being where the board firmware + OS are being read from.**
Instead of thinking of this as "Boot from SD or EMMC", think of these boot modes more like "Where the ZSBL (Zero^th^ Stage Bootloader) looks for the **firmware**: SPL (`u-boot`) `fw_payload` (OpenSBI)".
So, even when you have selected QSPI flash as the boot mode, it will read SPL and the `fw_payload` from there. But once the hardware initialization is complete, it will need a media to load the OS from. That can be either the SD Card, the EMMC flash or even an NVMe drive (support is not upstreamed yet!).
If the dip switches are set to boot either from the SD Card or from the EMMC flash, the partition table for the image that goes on the media in this case needs to have the following layout:
| **Partition** | **Start** | **End** | **Sectors** | **Size** | **Type** |
|---------------|-----------|---------|-------------|----------|------------------|
| 1 | 4096 | 8191 | 4096 | 2M | HiFive BBL |
| 2 | 8192 | 16383 | 8192 | 4M | HiFive FSBL |
| 3 | 16384 | 1064959 | 1048576 | 512M | EFI System |
| 4 | 1064960 | NA | NA | NA | Linux filesystem |
Since partitions 1 and 2 contain board specific files (SPL and `fw_payload` respectively), creating a generic image with this obviously not possible.
With a generic image, we can go with the conventional PC-style disk layout where we have a 512M partition for `/boot` and rest of the disk space for `/` (root). The board specific files are already on the board.
So now, two routes exist for us and I will lay out advantages and disadvantages for the generic route. You can assume the inverse to be true for the board-specific-image route.
## Advantages
1. Less storage + CDN cost.
2. Only one image to maintain, test and fix.
3. Less support headache.
4. Our image "should" work on SD, MMC and NVMe. (In theory, need to do more testing after uboot support is merged upstream.)
## Disadvantages
1. The firmware on the QSPI flash is not "prod ready". This obviously means that the user needs to be informed that their board firmware needs to be flashed. **IF DONE INCORRECTLY THE BOARD MAY BECOME A BRICK!** ("Prod ready" means any distro can depend on it to base their image off of.)
2. We need the user to have access to the Serial console to issue some uboot related commands that enable a boot order of 'sd emmc nvme pxe dhcp'. Not everyone has a USB-to-TTL cable though.
3. Adding features like GPU support (in the kernel), PCIe support (in u-boot), etc will be slower because, well, we are relying on upstream, not the vendor.