linux-kernel - Re: Does pci_map_single magicly work on > 32 bit address for 32bit capable device?

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Date:	Fri, 11 Dec 2009 18:38:09 -0600
From:	Robert Hancock <hancockrwd@...il.com>
To:	Maxim Levitsky <maximlevitsky@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Does pci_map_single magicly work on > 32 bit address for 32bit 
	capable device?

On Fri, Dec 11, 2009 at 6:18 PM, Maxim Levitsky <maximlevitsky@...il.com> wrote:
>> The kernel should set up an IOMMU (either hardware or software) mapping
>> for that memory so that the device can access it through an address
>> below 4GB. This is assuming it's a 64-bit kernel (on 32-bit, a kernel
>> memory address will always be below 4GB).
>
> What do you mean by software IOMMU?
>
> On my system there is no IOMMU present, so only way to ensure 32 bit
> address is to copy pages.
> pci_map_single could copy the data for write case, and pci_unmap_single
> for read case, but I now strongly doubt they do.

It does. See the swiotlb code. You should see some message on bootup like:

PCI-DMA: Using software bounce buffering for IO (SWIOTLB)
Placing 64MB software IO TLB between ffff880020000000 - ffff880024000000
software IO TLB at phys 0x20000000 - 0x24000000

> I am not sure at all that these functions will fail if too high address
> is specified....
>
> Also, very recently, I found that in
> Documentation/DMA-API.txt, there is a statement that says that
> pci_map_single fails for >32 bit memory address.

Where do you see this?

>
> I guess I just do a memcpy...

This isn't optimal because if there is a hardware IOMMU, the copy can
be avoided.
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