"Ubuntu got in touch with us to say ‘how can we stop you saying that Ubuntu runs on Raspberry Pi?’ Which I thought was pretty brutal, actually."
As a HW engineer he overlooked hidden software complications with ARMv6 chipset. The ARMv6 chipset is old, its virtually tagged/indexed caches have a plethora of cache aliasing issues that reflect to workarounds in linux kernel and shared libraries in userspace. You have to maintain an entire ARMv6 distro for this arch. I would personally not use anything non ARMv7, because all these issues are resolved in ARMv7.
So its quite natural Ubuntu did not want to allocate a team of 100k/year engineers for raspberry pi, and protect its brand also.
For the curious, if you map the same physical address to 2 virtual addresses, and both happen to be in the cache, you have 2 virtual entries in the cache for the same address, so which one to flush back to RAM is ambiguous. Its a problem specific to virtually addressed caches. Solution: a lot of restrictions to avoid aliasing and you flush the caches too often to avoid aliasing etc.
Thanks for explaining. Ubuntu's approach makes sense then. Wonder how much more expense v7 would have been.
I see the problem with Raspberry Pi being the software and the SDK. It is a very elegant and appealing piece of hardware but that only gets them there half way. The other is the manuals, the sdk, some kind of recipe exchange or cookbook for basic things.
Also, it looks like Arch Linux has stepped up to replace Ubuntu.
Once it has USB, Ethernet, and HDMI it is not your uncle's bare 8 bit micro-controller anymore. Yeah developers could sit and each write a different version of HDMI driver, but wouldn't be too much fun. One would expect there to be a nice SDK and libraries to access and make use of all those hardware features.
As a HW engineer he overlooked hidden software complications with ARMv6 chipset. The ARMv6 chipset is old, its virtually tagged/indexed caches have a plethora of cache aliasing issues that reflect to workarounds in linux kernel and shared libraries in userspace. You have to maintain an entire ARMv6 distro for this arch. I would personally not use anything non ARMv7, because all these issues are resolved in ARMv7.
So its quite natural Ubuntu did not want to allocate a team of 100k/year engineers for raspberry pi, and protect its brand also.
For the curious, if you map the same physical address to 2 virtual addresses, and both happen to be in the cache, you have 2 virtual entries in the cache for the same address, so which one to flush back to RAM is ambiguous. Its a problem specific to virtually addressed caches. Solution: a lot of restrictions to avoid aliasing and you flush the caches too often to avoid aliasing etc.