This looks like a configuration regression, see bug #1534647. Applied patch 'UBUNTU: [Config] CONFIG_ZONE_DMA=y for generic' which will appear in Ubuntu-4.8.0-19.21
This looks like a configuration regression, see bug #1534647. Applied patch 'UBUNTU: [Config] CONFIG_ZONE_DMA=y for generic' which will appear in Ubuntu-4.8.0-19.21