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

[Mark]

On Sun, January 24, 2016 21:42, Tim Gardner wrote:
> 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.

It seems there is some upstream discussion and an RFC patch (which did
seem to fix the problem for me). That patch probably won't be committed
as-is, but hopefully a solution will be found so this issue ceases to be a
problem.

http://www.spinics.net/lists/kernel/msg2170767.html

I don't know whether any solution will get backported to kernel 4.4, but
if not, perhaps backporting it could be one of the Ubuntu-specific kernel
changes.


Mark