linux-kernel - Re: [PATCH v2 3/5] sh/PCI: Pass GAPSPCI_DMA_BASE CPU address to dma_declare_coherent

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Date:	Wed, 07 May 2014 10:15:10 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	linux-doc@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Joerg Roedel <joro@...tes.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Liviu Dudau <Liviu.Dudau@....com>,
	linux-kernel@...r.kernel.org,
	James Bottomley <jbottomley@...allels.com>,
	linux-pci@...r.kernel.org, David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH v2 3/5] sh/PCI: Pass GAPSPCI_DMA_BASE CPU address to dma_declare_coherent_memory()

On Wednesday 07 May 2014 09:55:16 Arnd Bergmann wrote:
> On Tuesday 06 May 2014 16:48:33 Bjorn Helgaas wrote:
> > @@ -51,8 +53,12 @@ static void gapspci_fixup_resources(struct pci_dev *dev)
> >                 /*
> >                  * Redirect dma memory allocations to special memory window.
> >                  */
> > +               region.start = GAPSPCI_DMA_BASE;
> > +               region.end = GAPSPCI_DMA_BASE + GAPSPCI_DMA_SIZE - 1;
> > +               res.flags = IORESOURCE_MEM;
> > +               pcibios_bus_to_resource(dev->bus, &res, &region);
> >                 BUG_ON(!dma_declare_coherent_memory(&dev->dev,
> > -                                               GAPSPCI_DMA_BASE,
> > +                                               res->start,
> >                                                 GAPSPCI_DMA_BASE,
> >                                                 GAPSPCI_DMA_SIZE,
> >                                                 DMA_MEMORY_MAP |
> 
> Not sure if this is an improvement. I understand the intention, but
> it's actually possible for the offset to be different both ways:
> 
> Your patch applies the outbound mem_offset that was provided when
> registering the MMIO resource for the PCI host bridge. What the driver
> needs instead is the inbound DMA offset that gets applied by some
> host bridges that don't have a 1:1 mapping but also don't have an
> IOMMU. We know that on this particular platform, both are zero, so
> the original code works and it will keep working with your change,
> but I think it's a mistake anyway. I have seen both kinds of offsets
> in the past on real machines, but I am not aware of any where they
> are both nonzero and have the same value.

Hmm, looking at it again, it seem even weirder: the address we register
for DMA allocations is the same that gets passed into the PCI host
for MMIO resources. Something is very strange here. Still, I'd rather
not touch it at all, whatever it does.

	Arnd
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