4 ms·

Nothing about the Raspberry Pi’s video system is normal. Basically, disregard everything you’ve learned about KMS from the world of Raspberry Pi. It’s only rel

Nothing about the Raspberry Pi’s video system is normal. Basically, disregard everything you’ve learned about KMS from the world of Raspberry Pi. It’s only relevant to the quirkiness of the Raspberry Pi’s half-closed ecosystem. The F in fkms stands for “fake” because they created a quirky abstraction to fake KMS compatibility. The non-fake driver has been all sorts of problematic but it’s getting better. But none of this Raspberry Pi specific weirdness should be extrapolated to the KMS/DRM subsystem in general.

Syonyk 5y ago
> Nothing about the Raspberry Pi’s video system is normal. Nothing about the Raspberry Pi at all is "normal," and it's a very useful filter for people who have read about the various deep weeds of firmware/OS development and those who have actually gotten their hands dirty on a Pi - because the first group will express admiration about the "open and well documented hardware," and the second group will laugh at the first group and start cursing the various non-and-poorly-documented nonsense that's in the Pis. They're nice devices, and work well enough, but they are architecturally insane and unlike anything else except the previous Raspberry Pis that were hacked to to make the new ones. The Pi1/2/3 are more or less an (undocumented) GPU with an ARM core bolted on the side. The GPU boots the system, allows the ARM core some access to the DRAM (not at the same addresses as the GPU sees things, so be sure which version of memory addresses you're looking at), and most of the hardware control is done by asking the GPU to please do this thing for you through the mailbox interfaces. In the same way that userland applications make syscalls into the kernel, the kernel running on a Pi makes syscall-like requests into the GPU to do just about anything. The reason you see things like "All the interrupts are only ever on a single core" is because the interrupt controllers simply don't allow anything else. The Pi2 BCM2836 interrupt controller feels a little bit like an intern's project, using the BCM2835 bolted on the front as an input for the rest of the hardware. At least the Pi4 finally gets a GIC... They're fine, sort of... as long as you're OK with the only documentation for some of the hardware being "This Linux driver works with it" and "Well, people have reverse engineered most of it." But they're absolutely insane, architecturally, they're poorly documented, and this doesn't seem widely known.
- cagey 5y ago
  > The Pi1/2/3 are more or less an (undocumented) GPU with an ARM core bolted on the side. Is the Pi4 any different/better?
  - Syonyk 5y ago
    I don't know the Pi4 architecture deeply enough to offer an opinion on it, sorry. Ask me in a year or two and I'll likely know it a lot better.
  - monocasa 5y ago
    It's almost the same overall architecture, just a lot of pieces have been upgraded to be wider and faster, and some peripherals added.
  - KennyBlanken 5y ago
    Well, they fucked up their implementation of USB-PD with a rookie-level mistake, it took ages for them to support booting off USB, the 5V rail is noisy... Raspberry Pis were initially just a way for the Pi foundation to help Broadcom sell SoCs nobody wants anymore but they still had lots of stock/tooling for. Sort of like "hybrid" bicycles are how bike manufacturers milk more money out of hardtail (non-suspension) MTB frames.
- derefr 5y ago
  Doesn’t sound too insane; sounds like an architecture designed for real-time professing, where you’ve got an “IO processor” that boots the system and handles all the interrupt-heavy work, and an “application processor” that can be solely dedicated to your workload and will only be interrupted by the IO processor if it explicitly asks to be by first making a request to it. Very similar to previous-gen game consoles, e.g. the Wii U’s “Broadway”
  - froh 5y ago
    Does the pi allow to put own routines straight onto it's "IO processor"?
- 5y ago
jancsika 5y ago
Ah, thanks for that. > Basically, disregard everything you’ve learned about KMS from the world of Raspberry Pi. It’s only relevant to the quirkiness of the Raspberry Pi’s half-closed ecosystem. It's funny, I just posted a comment about being afraid to dig into KMS code for fear a wizard would show up and tell me I'm looking in a wrong or irrelevant place. Luckily, your comment preempts that process. And also luckily, I've only thrown flags at Chromium provided by other wizards so far. So my ignorance about KMS remains pure. :) > The non-fake driver has been all sorts of problematic but it’s getting better. So... are there like two different teams, one Pi-specific 'f', one general 'not f', racing to see whose module will become stable first? Do both teams stream on Twitch? If not they should.
deleted 5y ago
[deleted]

Syonyk 5y ago

> Nothing about the Raspberry Pi’s video system is normal. Nothing about the Raspberry Pi at all is "normal," and it's a very useful filter for people who have read about the various deep weeds of firmware/OS development and those who have actually gotten their hands dirty on a Pi - because the first group will express admiration about the "open and well documented hardware," and the second group will laugh at the first group and start cursing the various non-and-poorly-documented nonsense that's in the Pis. They're nice devices, and work well enough, but they are architecturally insane and unlike anything else except the previous Raspberry Pis that were hacked to to make the new ones. The Pi1/2/3 are more or less an (undocumented) GPU with an ARM core bolted on the side. The GPU boots the system, allows the ARM core some access to the DRAM (not at the same addresses as the GPU sees things, so be sure which version of memory addresses you're looking at), and most of the hardware control is done by asking the GPU to please do this thing for you through the mailbox interfaces. In the same way that userland applications make syscalls into the kernel, the kernel running on a Pi makes syscall-like requests into the GPU to do just about anything. The reason you see things like "All the interrupts are only ever on a single core" is because the interrupt controllers simply don't allow anything else. The Pi2 BCM2836 interrupt controller feels a little bit like an intern's project, using the BCM2835 bolted on the front as an input for the rest of the hardware. At least the Pi4 finally gets a GIC... They're fine, sort of... as long as you're OK with the only documentation for some of the hardware being "This Linux driver works with it" and "Well, people have reverse engineered most of it." But they're absolutely insane, architecturally, they're poorly documented, and this doesn't seem widely known.

cagey 5y ago
> The Pi1/2/3 are more or less an (undocumented) GPU with an ARM core bolted on the side. Is the Pi4 any different/better?
- Syonyk 5y ago
  I don't know the Pi4 architecture deeply enough to offer an opinion on it, sorry. Ask me in a year or two and I'll likely know it a lot better.
- monocasa 5y ago
  It's almost the same overall architecture, just a lot of pieces have been upgraded to be wider and faster, and some peripherals added.
- KennyBlanken 5y ago
  Well, they fucked up their implementation of USB-PD with a rookie-level mistake, it took ages for them to support booting off USB, the 5V rail is noisy... Raspberry Pis were initially just a way for the Pi foundation to help Broadcom sell SoCs nobody wants anymore but they still had lots of stock/tooling for. Sort of like "hybrid" bicycles are how bike manufacturers milk more money out of hardtail (non-suspension) MTB frames.
derefr 5y ago
Doesn’t sound too insane; sounds like an architecture designed for real-time professing, where you’ve got an “IO processor” that boots the system and handles all the interrupt-heavy work, and an “application processor” that can be solely dedicated to your workload and will only be interrupted by the IO processor if it explicitly asks to be by first making a request to it. Very similar to previous-gen game consoles, e.g. the Wii U’s “Broadway”
- froh 5y ago
  Does the pi allow to put own routines straight onto it's "IO processor"?
5y ago

jancsika 5y ago

Ah, thanks for that. > Basically, disregard everything you’ve learned about KMS from the world of Raspberry Pi. It’s only relevant to the quirkiness of the Raspberry Pi’s half-closed ecosystem. It's funny, I just posted a comment about being afraid to dig into KMS code for fear a wizard would show up and tell me I'm looking in a wrong or irrelevant place. Luckily, your comment preempts that process. And also luckily, I've only thrown flags at Chromium provided by other wizards so far. So my ignorance about KMS remains pure. :) > The non-fake driver has been all sorts of problematic but it’s getting better. So... are there like two different teams, one Pi-specific 'f', one general 'not f', racing to see whose module will become stable first? Do both teams stream on Twitch? If not they should.

deleted 5y ago

[deleted]