Re: [RFC PATCH v5 3/8] iommu: add a new capable IOMMU_CAP_MERGING

[Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>]

On Fri, Jun 07, 2019 at 05:41:56AM +0000, Yoshihiro Shimoda wrote:
> > bool blk_can_use_iommu_merging(struct request_queue *q, struct device *dev)
> > {
> > 	if (!IOMMU_CAN_MERGE_SEGMENTS(dev))
> > 		return false;
> 
> As Robin mentioned, all IOMMUs can merge segments so that we don't need
> this condition, IIUC. However, this should check whether the device is mapped
> on iommu by using device_iommu_mapped().

There are plenty of dma_map_ops based drivers that can't merge segments.
Examples:

 - arch/ia64/sn/pci/pci_dma.c
 - arch/mips/jazz/jazzdma.c
 - arch/sparc/mm/io-unit.c
 - arch/sparc/mm/iommu.c
 - arch/x86/kernel/pci-calgary_64.c

Nevermind the diret mapping, swiotlb and other weirdos.

> 
> > 	blk_queue_virt_boundary(q, IOMMU_PAGE_SIZE(dev));
> > 	blk_queue_max_segment_size(q, IOMMU_MAX_SEGMENT_SIZE(dev));
> 
> By the way, I reported an issue [1] and I'm thinking dima_is_direct() environment
> (especially for swiotlb) is also needed such max_segment_size changes somehow.
> What do you think?
> 
> [1]
> https://marc.info/?l=linux-block&m=155954415603356&w=2

That doesn't seem to be related to the segment merging.  I'll take
a look, but next time please Cc the author of a suspect commit if
you already bisect things.