linux-kernel - Re: [RFC 0/3] iommu: Reserved regions for IOVAs beyond dma

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Date:   Mon, 5 Oct 2020 15:08:52 +0200
From:   Christoph Hellwig <hch@....de>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Auger Eric <eric.auger@...hat.com>, joro@...tes.org,
        iommu@...ts.linux-foundation.org, robin.murphy@....com,
        dwmw2@...radead.org, eric.auger.pro@...il.com,
        linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        shameerali.kolothum.thodi@...wei.com,
        Jean-Philippe Brucker <jean-philippe@...aro.org>, hch@....de
Subject: Re: [RFC 0/3] iommu: Reserved regions for IOVAs beyond dma_mask
 and iommu aperture

On Mon, Oct 05, 2020 at 11:44:10AM +0100, Lorenzo Pieralisi wrote:
> > I see that there are both OF and ACPI hooks in pci_dma_configure() and
> > both modify dev->dma_mask, which is what pci-sysfs is exposing here,
> > but I'm not convinced this even does what it's intended to do.  The
> > driver core calls this via the bus->dma_configure callback before
> > probing a driver, but then what happens when the driver calls
> > pci_set_dma_mask()?  This is just a wrapper for dma_set_mask() and I
> > don't see anywhere that would take into account the existing
> > dev->dma_mask.  It seems for example that pci_dma_configure() could
> > produce a 42 bit mask as we have here, then the driver could override
> > that with anything that the dma_ops.dma_supported() callback finds
> > acceptable, and I don't see any instances where the current
> > dev->dma_mask is considered.  Am I overlooking something? 
> 
> I don't think so but Christoph and Robin can provide more input on
> this - it is a long story.
> 
> ACPI and OF bindings set a default dma_mask (and dev->bus_dma_limit),
> this does not prevent a driver from overriding the dev->dma_mask but DMA
> mapping code still takes into account the dev->bus_dma_limit.
> 
> This may help:
> 
> git log -p 03bfdc31176c

This is at best a historic artefact.  Bus drivers have no business
messing with the DMA mask, dev->bus_dma_limit is the way to communicate
addressing limits on the bus (or another interconnect closer to the CPU).