Supporting segment sizes smaller than the page size

[Bart Van Assche <bvanassche@xxxxxxx>]

Hi Jens, Christoph and Ming,

As we know the Linux kernel block layer does not support DMA segment 
sizes that are smaller than the page size. From block/blk-settings.c:

void blk_queue_max_segment_size(struct request_queue *q,
                                unsigned int max_size)
{
	if (max_size < PAGE_SIZE) {
		max_size = PAGE_SIZE;
		printk(KERN_INFO "%s: set to minimum %d\n",
		       __func__, max_size);
	}

	/* see blk_queue_virt_boundary() for the explanation */
	WARN_ON_ONCE(q->limits.virt_boundary_mask);

	q->limits.max_segment_size = max_size;
}

I have been asked to add support for DMA segment sizes that are smaller 
than the page size to support a UFS controller with a maximum DMA 
segment size of 4 KiB and a page size > 4 KiB (my understanding of the 
JEDEC UFS host controller specification is that UFS host controllers 
should support a maximum DMA segment size of 256 KiB). Does anyone want 
to comment on this before I start working on a patch series?

Thanks,

Bart.