linux-kernel - Re: d63e2e1f3df breaks sparc/T5-8

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Message-ID: <20150403154526.GB10892@google.com>
Date:	Fri, 3 Apr 2015 10:45:26 -0500
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	David Miller <davem@...emloft.net>
Cc:	bjorn@...gaas.com, bjorn.helgaas@...il.com,
	sparclinux@...r.kernel.org, linux-pci@...r.kernel.org,
	yinghai@...nel.org, david.ahern@...cle.com,
	linux-kernel@...r.kernel.org
Subject: Re: d63e2e1f3df breaks sparc/T5-8

On Sun, Mar 29, 2015 at 11:32:50AM -0700, David Miller wrote:
> From: Bjorn Helgaas <bjorn.helgaas@...il.com>
> Date: Sun, 29 Mar 2015 08:30:40 -0500
> 
> > Help me understand the sparc64 situation: are you saying that BAR
> > addresses, i.e., MMIO transactions from a CPU or a peer-to-peer DMA can be
> > 64 bits, but a DMA to main memory can only be 32 bits?
> > 
> > I assume this would work if we made dma_addr_t 64 bits on sparc64.  What
> > would be the cost of doing that?
> 
> The cost is 4 extra bytes in every datastructure, kernel wide, that
> stores DMA addresses.

That much is fairly obvious.  What I don't know is how much difference this
makes in the end.

> Don't use DMA addresses for PCI addresses.  They are absolutely not
> the same, especially when an IOMMU is always present because in that
> case all DMA addresses are virtual and exist in a different realm
> and set of constraints/restrictions.

I'm still trying to figure out a clear description of how a DMA address is
different from a PCI address.  If you capture a transaction with a PCI
analyzer, I don't think you can tell a DMA address from a PCI address
unless you know how bridge windows are programmed.  Even then, I'm not sure
you can tell a CPU-generated PCI address from a DMA address in a
device-generated peer-to-peer transaction.

Bjorn
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