On Tue, Nov 27, 2012 at 01:09:46PM +0100, Jens Axboe wrote: > On 2012-11-27 01:35, Laine Walker-Avina wrote: > > Hi all, > > > > We are experiencing an issue with doing direct IO to a NVMe device I'm > > helping to develop. Every so often, the physical address given by > > sg_dma_address() is aligned to 0x800 instead of 0x1000 as specified by > > blk_queue_dma_alignement(queue, 4095) when the queue is initialized.
FYI, this is a modification to the driver that Laine has made; presumably for a limitation of the prototype hardware he's working with. The NVMe spec requires the device to be able to do I/Os to 4 byte boundaries. Laine, when this occurs, what is the alignment of 'offset' in the sg entry you're looking at? If userspace is passing in an unaligned address, I don't think there's anything we do to try to align it. > > The request is also split over multiple segments to make up for the > > missing space (eg: for a 4k IO it's split into two segments 2k in > > size, and for an 8k IO it's split into 3 segments--2k,4k,2k). Our > > design requires the physical segments given to the device be aligned > > to 4k boundaries and be multiples of 4k in size. When not doing direct > > IO the physical addresses appear to always be 4k aligned as expected. > > One possible issue is the kernel we're primarily testing against is > > 2.6.32-220 from CentOS, but we have observed similar behavior from a > > vanilla 3.3 kernel as well. Any help would be greatly appreciated. > > I'm assuming you set the hardware sector size to 4k as well? > > -- > Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
