RE: [RFC PATCH v6 5/5] mmc: queue: Use bigger segments if IOMMU can merge the segments

[Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>]

Hi Christoph,

> From: Christoph Hellwig, Sent: Monday, June 17, 2019 3:54 PM
> 
> On Mon, Jun 17, 2019 at 06:46:33AM +0000, Yoshihiro Shimoda wrote:
> > > can_merge seems a little too generic a name to me.  Maybe can_iommu_merge?
> >
> > I'll fix the name. Also, only the device_iommu_mapped() condition wiil cause
> > a problem on iommu=pt [1]. So, I'll add another condition here.
> 
> Instead of adding another condition here I think we need to properly
> abstract it out in the DMA layer.  E.g. have a

Thank you for your comment and sample code! I'll add such functions
on next patch series.

Best regards,
Yoshihiro Shimoda

> unsigned long dma_get_merge_boundary(struct device *dev)
> {
> 	const struct dma_map_ops *ops = get_dma_ops(dev);
> 
> 	if (!ops || !ops->get_merge_boundary)
> 		return 0; /* can't merge */
> 	return ops->get_merge_boundary(dev);
> }
> 
> and then implement the method in dma-iommu.c.
> 
> blk_queue_can_use_iommu_merging then comes:
> 
> bool blk_queue_enable_iommu_merging(struct request_queue *q,
> 		struct device *dev)
> {
> 	unsigned long boundary = dma_get_merge_boundary(dev);
> 
> 	if (!boundary)
> 		return false;
> 	blk_queue_virt_boundary(q, boundary);
> 	return true;
> }