linux-kernel - Re: [PATCH] DMA-API: Change dma_declare_coherent_memory() CPU address to phys_addr

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Date:	Tue, 6 May 2014 07:18:06 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	James Bottomley <jbottomley@...allels.com>
Cc:	"rdunlap@...radead.org" <rdunlap@...radead.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"arnd@...db.de" <arnd@...db.de>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] DMA-API: Change dma_declare_coherent_memory() CPU address
 to phys_addr_t

On Mon, May 5, 2014 at 8:42 PM, James Bottomley
<jbottomley@...allels.com> wrote:
> On Mon, 2014-05-05 at 17:01 -0600, Bjorn Helgaas wrote:
>> On Fri, May 02, 2014 at 10:42:18AM +0200, Arnd Bergmann wrote:
>
>> > I don't know about NCR_Q720, but all others are only used on machines
>> > where physical addresses and bus addresses are in the same space.
>>
>> In general, the driver doesn't know whether physical and bus addresses
>> are in the same space.  At least, I *hope* it doesn't have to know,
>> because it can't be very generic if it does.
>
> The API was designed for the case where the memory resides on a PCI
> device (the Q720 case), the card config gives us a bus address, but if
> the system has an IOMMU, we'd have to do a dma_map of the entire region
> to set up the IOMMU before we can touch it.  The address it gets back
> from the dma_map (the dma_addr_t handle for the IOMMU mapping) is what
> we pass into dma_declare_coherent_memory().

The IOMMU (if any) is only involved for DMA to system memory, and
there is no system memory in this picture.  The device does DMA to its
own memory; no dma_map is required for this.  We use
dma_declare_coherent_memory() to set things up so the CPU can also do
programmed I/O to the memory.

> The reason it does an
> ioremap is because this IOMMU mapped address is now physical to the CPU
> and we want to make the region available to virtual space.  Essentially
> the memory the allocator hands out behaves as proper virtual memory but
> it's backed by physical memory on the card behind the PCI bridge.

Yep, the programmed I/O depends on the ioremap().  But I don't think
it depends on any IOMMU mapping.

> I'm still not that fussed about the difference between phys_addr_t and
> dma_addr_t, but if the cookie returned from a dma_map is a dma_addr_t
> then that's what dma_declare_coherent_memory() should use.  If it's a
> phys_addr_t, then likewise.
>
> James
>
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