[PATCH Xenial 4.4] UBUNTU: [Config] CONFIG_ZONE_DEVICE=y for amd64

[Tim Gardner]

On 01/23/2016 10:47 AM, Mark wrote:
> On Sat, January 23, 2016 14:13, Tim Gardner wrote:
>> In the event that upstream does not come up with a solution, how about
>> if we leave ZONE_DMA enabled in the low-latency flavour ? After all, it
>> seems most of the devices that this affects are audio related (except
>> for the parallel port).
>
> Another point of information. According to a post in fa.linux.kernel from
> 2007, some Broadcom NICs may need ZONE_DMA.
>
> https://groups.google.com/forum/#!original/fa.linux.kernel/bXnCP9QIlMI/V6pLHumoe0oJ
>
> "Just because you don't need ISA, doesn't mean you don't need
> ZONE_DMA. Various PCI cards have limitations on what addresses they
> can DMA to (24 bits on some old PCI sound cards iirc, and more recently
> a 30 bit DMA limitation on some Broadcom networking equipment.)"
>
> Another thread mentions the floppy controller, so systems with native
> floppy drives could be another casualty:
> https://groups.google.com/forum/#!original/fa.linux.kernel/rM_PYJc9ExE/brVe--Xr0FgJ
>
> Is there an easy way to grep the kernel source tree to get a list of all
> drivers which depend on ZONE_DMA? I tried that in the drivers subdirectory
> and came up with
> net/wireless/b43/dma.c
> net/wireless/b43legacy/dma.c
>
> Grepping for GFP_DMA returned many more results; someone more
> knowledgeable than me might be able to say whether any are relevant.
>
>
> Mark
>
>

This seems to be a bit of an intractable problem, and I don't see an 
upstream solution any time soon (if ever). Therefore, I've implemented a 
compromise as detailed in 
https://bugs.launchpad.net/ubuntu/xenial/+source/linux/+bug/1534647/comments/3

Regardless of the impact on these legacy devices, I feel strongly that 
NVDIMMs must be supported on amd64 servers for 16.04. Also, I do not 
think this regression in support for legacy devices warrants a new amd64 
flavour given the lowlatency compromise.

rtg
-- 
Tim Gardner tim.gardner at canonical.com