RE: [RFC PATCH v6 3/5] block: add a helper function to merge the segments by an IOMMU

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

Hi Robin,

> From: Robin Murphy, Sent: Friday, June 14, 2019 6:55 PM
> 
> On 13/06/2019 11:20, Yoshihiro Shimoda wrote:
<snip>
> > +bool blk_queue_can_use_iommu_merging(struct request_queue *q,
> > +				     struct device *dev)
> > +{
> > +	struct iommu_domain *domain;
> > +
> > +	/*
> > +	 * If the device DMA is translated by an IOMMU, we can assume
> > +	 * the device can merge the segments.
> > +	 */
> > +	if (!device_iommu_mapped(dev))
> 
> Careful here - I think this validates the comment I made when this
> function was introduced, in that that name doesn't necesarily mean what
> it sounds like it might mean - "iommu_mapped" was as close as we managed
> to get to a convenient shorthand for "performs DMA through an
> IOMMU-API-enabled IOMMU". Specifically, it does not imply that
> translation is *currently* active; if you boot with "iommu=pt" or
> equivalent this will still return true even though the device will be
> using direct/SWIOTLB DMA ops without any IOMMU translation.

Thank you for your comments. I understood the mean of "iommu_mapped" and
this patch's condition causes a problem on iommu=pt.
So, I'll add and additional condition like
"domain->type == IOMMU_DOMAIN_DMA" to check whether the translation is
currently active on the domain or not.

Best regards,
Yoshihiro Shimoda