[DragonFlyBSD - Submit #2883] Allocate DMA buffers in first 4GB

bugtracker-admin at leaf.dragonflybsd.org bugtracker-admin at leaf.dragonflybsd.org
Sun Jan 17 08:46:16 PST 2016

Issue #2883 has been updated by xenu.

Thank you for your reply, now it's a lot more clear to me! Still, it looks like its obfuscated and definitely could use more comments in code :)

So the driver itself should request 32 bit memory, and kmalloc() thing is an optimization. Got it. But I still don't fully understand the purpose of two-tries-of-kmalloc construction. It looks dodgy at best, without comments I can only assume that it  either has certain, unclear assumptions on undocumented kmalloc() behavior or it just tries to get lucky twice. I can be wrong, but it just doesn't look like a very good design.

Also, I still think it's simpler to forbid allocating above 32-bits when there is no IOMMU support (just like in OpenBSD), but that's not something I'm going to argue about, as there are good reasons to not like this idea :)

I'll find offending driver when I'll have chance to boot dfly again.

Submit #2883: Allocate DMA buffers in first 4GB

* Author: xenu
* Status: New
* Priority: Normal
* Assignee: 
* Category: 
* Target version: 

I tried to boot DragonFlyBSD on my AMD notebook, unfortunately it doesn't work. The problem is that on modern AMD systems, if you don't properly support AMD IOMMU, you have to allocate DMA buffers in the first 4 gigabytes of memory. That's how OpenBSD (and probably others) is doing this.

I've attached patch that fixes it in busdma_machdep.c. With this patch applied, DragonFlyBSD boots fine on my machine (and probably other modern AMD computers).

As a bonus, it simplifies code a bit, but I'm not really sure if it's a good thing. I just don't quite understand why this code wasn't using contigmalloc() all along, which seems to be more fitting for this kind of stuff.

Looks like previous code used kmalloc() when needed memory block was small enough, and then prayed for proper alignment. It retried if alignment wouldn't turn out to be OK. If requested memory block was larger, it would've used contigmalloc(). It just doesn't feel right.

I'm no expert on this stuff, so any comments are welcome!

dma_4gb.patch (3.4 KB)

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