DeVault: GitHub Copilot and open source laundering [LWN.net]

DeVault: GitHub Copilot and open source laundering

Posted Jun 26, 2022 19:40 UTC (Sun) by mjg59 (subscriber, #23239)
In reply to: DeVault: GitHub Copilot and open source laundering by flussence
Parent article: DeVault: GitHub Copilot and open source laundering

> the R200/(reverse-engineered)R300 driver also used to be blob-free

The DRM side of the R2/3/400 driver always required a firmware blob - but for a long time it was just embedded inside the kernel driver, so wasn't user visible. My recollection (which seems to be supported by the driver, but it's been a long time since I looked at this properly so I could be wrong) is that a bunch of the 2D acceleration in that driver depended on DRM, so effectively the 2D driver also had a blob dependency if you wanted it to work properly.

The difference between -radeonhd and -ati as far as reliance on firmware goes was that the defined interface to various pieces of card functionality was to execute interpreted scripts present in the card flash. These scripts didn't do anything that the driver couldn't, so you could absolutely reimplement that functionality in the driver - the problem is that card vendors used these scripts as a way to abstract hardware differences (eg, using RAM from different vendors with different timing constraints), and ignoring Atom would mean having to have card-specific data in the driver before that card would work correctly. A hybrid approach is to use Atom for data but not for code, but even then there are still risks due to the fact that the defined interface is the scripts and not the data tables. A card vendor could modify the way the script interpreted the tables (or even hardcode stuff directly into the script) and again you'd need card-specific knowledge to avoid that. -radeonhd spent a while trying to avoid executing any Atom code, but effectively relied on it anyway - it couldn't program the card from cold and so depended on the system firmware having executed the scripts before it ran. In any case, support for executing Atom code (including running the ASIC init function) was added to -radeonhd by September of 2007.

Looking at the initial commits to support r500 in the -ati driver, I think the only time it would ever call int10 is if the card was entirely uninitialised. -radeonhd would do exactly the same if it was configured without support for doing Atom-based init.

to post comments

DeVault: GitHub Copilot and open source laundering

Posted Jun 29, 2022 10:12 UTC (Wed) by flussence (guest, #85566) [Link] (2 responses)

That first paragraph's a letdown, but...

I learned something today. Thanks for explaining all that to my dumb ass.

This is the kind of thing that makes me keep my subscription to the site going.

DeVault: GitHub Copilot and open source laundering

Posted Jun 29, 2022 10:31 UTC (Wed) by mjg59 (subscriber, #23239) [Link] (1 responses)

The easiest way to think about Atom is like ACPI - it's a lump of bytecode that's interpreted in kernel context, and which talks to hardware the kernel could already talk to. We could rip out the ACPI interpreter in the kernel and hardcode knowledge of every singly ACPI board instead, and we'd have successfully replaced non-free code with free code. But we'd also have a kernel that didn't boot on a bunch of new systems until people had reverse engineered the relevant ACPI code and reimplemented it, and so nobody has seriously suggested doing that.

There's tradeoffs. I'd love to avoid having to rely on non-free code to make hardware work, and I'm not going to criticise people for writing drivers that do that. But the reality is that any such driver is going to work less well than a driver that uses the defined interface to call non-free firmware (in much the same way that we call into non-free UEFI code to set boot variables these days), and so there's value in the driver that calls non-free code existing, and also it's unsurprising that distros would pick the one that works with more hardware.

Luc's priorities on -radeonhd were probably based on his experience with the VIA chipsets that were extremely limited by what the BIOS permitted (and yeah it turns out that not being able to set any modes other than those that are hardcoded in the BIOS is not good!), but the outcome was also that his driver for those chipsets simply didn't work on all hardware - I had a VIA-based laptop that would just give a black screen with his driver, because the BIOS didn't match his expectations. To be completely fair, on Radeon I hit some similar constraints when I was researching reclocking the RAM for power management - the Atom scripts simply took too long, so I took out a bunch of the wait statements and hardcoded those into the kernel and it was great, and then after a couple of days of uptime the card would wedge during a reclock and also it didn't work on all hardware, so it turns out there was a reason that those were there in the first place. So I absolutely understand the desire to have native code for all of this, but also in the absence of vendors providing explicit contracts about hardware behaviour, a driver that doesn't use the defined interfaces is inherently going to break things.

DeVault: GitHub Copilot and open source laundering

Posted Jun 29, 2022 11:31 UTC (Wed) by farnz (subscriber, #17727) [Link]

It's also worth noting in this context that part of the reason to have vendor scripts of some form (be they AtomBIOS, ACPI or others) is that power delivery changes can't happen instantly - and different board manufacturers will have done different transient analysis to determine what their hardware can reliably support. The results of that analysis (whether it's a "rule of thumb" assessment or a proper calculation) need communicating to the driver somehow - and a small scripting language is as good as any other way to deal with it, especially since the edge cases get complex if you're doing a per-board calculation based on measuring the final system during post-manufacture testing of a board.

That said, given the quality of some vendor code, I understand Luc's reluctance to trust it - I've encountered one vendor who asserted that the CPU would detect an OUT 0xCF8, EAX instruction in userspace and then ensure that nothing else accessed PCI configuration space until the same userspace process executed either OUT 0xCFC, EAX or IN EAX, 0xCFC later, on the basis that if the CPU didn't do that, it would be possible for their userspace driver to crash. I'm not even sure how this could work under Linux…