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ARM Is a Promising Platform But Needs To Learn From the PC
jbrodkin writes "Linux and ARM developers have clashed over what's been described as a 'United Nations-level complexity of the forks in the ARM section of the Linux kernel.' Linus Torvalds addressed the issue at LinuxCon this week on the 20th anniversary of Linux, saying the ARM platform has a lot to learn from the PC. While Torvalds noted that 'a lot of people love to hate the PC,' the fact that Intel, AMD, and hardware makers worked on building a common infrastructure 'made it very efficient and easy to support.' ARM, on the other hand, 'is missing it completely,' Torvalds said. 'ARM is this hodgepodge of five or six major companies and tens of minor companies making random pieces of hardware, and it looks like they're taking hardware and throwing it at a wall and seeing where it sticks, and making a chip out of what's stuck on the wall.'"
What is a desktop in the PC world is your SOC in the embedded world. It even comes with RAM and Flash (not on chip, but on package), if you want to.
The difference is that the PC environment has over a long time filtered down to a few typical devices for each type. Your network hardware is probably Realtek, or maybe Intel or an embedded AMD chip. You graphics card is NVidia, AMD or Intel. Your mouse does not matter, because it always talk USB HID etc.
In the ARM world, you also have standard components, but every integrator makes tiny (and usually pointless) changes that render them incompatible on the software level. Linus is right - this is neither necessary nor sustainable. It is one of the reasons that you can get software updates for a 5 year old PC, but not for a 6 months old smartphone.
They're not trying to cut corners for the hell of it, but for performance, power usage, and other actual engineering reasons.
You just cant build smartphones and tablets with that same common architecture, or else you're adding too many chips and circuits you don't need.
It's no big deal that PC's ship with empty PCI slots and huge chunks of the bios and chipset that are never used but rarely. (Onboard raid, ECC codes, so on and so on), but when you're trying to put together a device as trim and minimalist as possible, you're going to end up with something slightly different for each use case.
Microkernel versus monolithic kernel has nothing to do with board support packages.
Linux has the equivalent of "board support packages" -- they can be as small as one file, but are more often just a handful: a C file that describes memory and I/O mappings and other peripherals that cannot be safely detected at runtime, sometimes a default configuration (defconfig) file, and maybe some other pretty small driver-like files that manage some of the mess that Linus was talking about. (For example, the BeagleBoard has three C files: one to define the board, one to manage LCD video configuration, and one for audio setup; it shares a defconfig with every other board using an OMAP2/3/4 CPU.)
That is in sharp contrast to my experience with commercial RTOSes, where a BSP might consist of a dozen C source and header files, plus another half-dozen configuration files. For the boards I have used, Linux has the smallest set of board-specific files, a microkernel RTOS has the next smallest, and a Unix-based RTOS has the largest. Linux doesn't call its board-specific file sets BSPs because they are (a) too small to really call a "package" and (b) not controlled and shipped separately. (Linux is not about locking down what the end user can do, so there would be no point in having BSPs for officially supported boards.)
Exactly.
The problem is not that adding support to a new board in Linux is too hard, in fact, it's almost the opposite. There are already tens of slightly incompatible boards to support, and every time a company makes a new one, they don't even try to stick to any standard (not that there even *exists* a real standard), since it's very easy to just add new code to Linux. See this LKML thread for Linus's description of the problem from some time ago.
Using a microkernel doesn't help at all; you still have to code for all of the slight incompatibilities, regardless of whatever differences in logical organization.