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raspberrypi / usbboot

Raspberry Pi USB booting code, moved from tools repository
Apache License 2.0
905 stars 229 forks source link

Fix compatibility with big endian hosts #70

Open zeldin opened 3 years ago

zeldin commented 3 years ago

The Pi bootcode expects the boot_message.length field to be sent in little endian, and sends the message.command in little endian. Make sure they are converted from whatever the host is using.

pelwell commented 3 years ago

Out of curiosity, what is the use case that triggered this PR? You don't see much big-endian hardware these days.

zeldin commented 3 years ago

@pelwell My main desktop computer is a Talos II (POWER9). But I also happen to run my Raspberry Pi 3 in big endian. :-) https://github.com/zeldin/linux-1/releases Now that I have a CM4 IO board with a PCIe slot, I hope to be able to debug the PCIe driver so that I can run Pi 4 in big endian mode as well...

pelwell commented 3 years ago

Having grown up on a 68000, little-endian was an adjustment, but I prefer it now.

usbboot isn't a paragon of programming style, but I think a better fix would be to declare the length field of bootmsg as an array of 4 uint8_t/unsigned chars and add another global variable - bootmsg_length, say - that holds the value in the natural endianness. You then only need to serialise just after receiving or just before sending, which doesn't need the union since you are working with an array.

zeldin commented 3 years ago

Sure. That also covers the case if int has a different size than 32 bits. I initially assumed you wanted to stay with C90 because I didn't see an include of <stdint.h> and the length/command fields were int rather than int32_t, but I do see now that C99 types are used in other places of main.c. I'll add the include line while I'm at it. :smile: