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

[Mark]

On Sat, January 23, 2016 14:13, Tim Gardner wrote:
> On 01/22/2016 07:56 AM, Mark wrote:
>> On Tue Jan 5 15:00:58 UTC 2016, Tim Gardner wrote:
>>
>>> In order for ZONE_DEVICE to be enabled for amd64 you have to set
>> ZONE_DMA=n (which is a change
>>>from our current annotation policy). In effect, this precludes devices
>> that can only DMA from
>>> memory addresses below 16MB. Does anyone think this is a bad idea ?
>> ZONE_DEVICE is a prequisite
>>> for supporting NVDIMM devices which I believe we'll start seeing more
>>> of in the near future.
>>
>> This change breaks PC parallel port DMA and will affect other ISA &
>> PCMCIA devices too I guess.
>>
>> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1536813
>> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1534647
>> https://bugzilla.kernel.org/show_bug.cgi?id=110931
>>
>> Best solution would be to get upstream to not break ZONE_DMA of course.
>>  From the commit message
>> (https://lists.01.org/pipermail/linux-nvdimm/2015-August/001810.html):
>> "However, since we have run out of ZONES_SHIFT bits this functionality
>> currently depends on sacrificing ZONE_DMA."
>>
>> If that isn't feasible, I'd suggest not shipping the default kernel with
>> ZONE_DEVICE, but perhaps make it available in a PPA for those that
>> want/need it.
>
> 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).

Interestingly, most (all??) affected sound cards listed at
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1536813 are PCI, not
ISA. I wonder if any other PCI devices would be affected. Or perhaps the
reason for PCI sound cards being affected relates to Sound Blaster
compatibility.

Having ZONE_DMA enabled in low-latency would be better than not having any
easily-installable kernel with ZONE_DMA.

There would still be out-of-the-box problems; for example user boots
Ubuntu DVD, wants to print or finds their system has no sound (playing
MP3s or YouTube experience impacted). But if the problem is
well-documented at least the user might try to install Ubuntu instead of
just trying another distro. An alternative could be to add the low-latency
kernel to the live DVD image. Perhaps not too desirable due to the
increased image size. Or perhaps build a ZONE_DMA-enabled kernel for use
on the live DVD? Or offer ZONE_DMA and non-ZONE_DMA kernels on the
alternate image? [I'm guessing there isn't much appetite for maintaining
another kernel variant, given how the non-PAE 32-bit kernels went away.]

Apart from that, would the low-latency kernel typically give reduced
battery life and louder fan noise due to increased idle power on a laptop?


Mark