Re: PowerPC PCI DMA issues (prefetch/coherency?)

From: Michael S. Zick
Date: Tue Jun 16 2009 - 10:16:40 EST

On Tue June 16 2009, Chris Pringle wrote:
> Hello All,
> We're developing on a Freescale MPC8272 and are having some nasty
> problems with PCI bus mastering and data corruption.
> We have some custom hardware that is bus mastering, reading data from
> the CPUs memory for it's own use. Most of the time, the data is correct,
> however occasionally we are seeing data that appears to be from
> somewhere else in memory (usually memory that has already been read by
> the PCI device). The problem looks like stale data on the PCI bridge
> prefetch buffers or a cache coherency problem, but we've been unable to
> come up with a solution to our problem. It is my understanding that it
> shouldn't be a cache coherency problem as the CPU cache should be
> snooped as the data is read from memory. Even if it were an issue, the
> pci_map_sg* functions should have sorted out any cache coherency issues
> before the DMA operation started.
> I've not been able to find anything on the Freescale data sheet that
> provides any way of flushing the prefetch cache on the PCI bridge. We've
> done a bit of experimenting, and found that turning off prefetch appears
> to solve (or possibly mask?) the problem (at the expensive of massive
> performance problems). I've also tried DMA'ing two adjacent userspace
> buffers in memory (from the same page), and see corruption on the second
> buffer. If I populate both buffers, then DMA them both, the data is
> fine. If I populate the first, DMA the first, then populate the second
> and DMA the second, corruption occurs at the start of the second buffer.
> If I add 8-32 bytes of padding between the buffers, the problem goes away.
> The PCI spec says that the PCI bridge is supposed to flush any data from
> it's prefetch buffers that are not read by the bus master, so
> technically, this isn't supposed to happen.
> I've tried making sure that buffers are cache line (and page) aligned,
> and are multiples of cache lines, but it's made no difference. PIO mode
> works fine, and I've checked the data with the CPU just before, and
> immediately after the DMA and the driver sees no data integrity issues.
> There are memory write barriers just before the DMA start, so all the
> registers should be correct before the DMA starts.
> For background info, the device doing the bus mastering is a Xilinx
> Virtex 5 FPGA. We've monitored the data as it comes off the PCI bus
> using ChipScope - so the firmware should not be manipulating the data in
> any way.
> We have some hardware/firmware/drivers that has a lot of common code
> that runs on an x86 platform (as opposed to powerpc), and that works
> without any issues whatsoever.
> Has anyone got any ideas what this might be? Does anyone of know issues
> with PCI bridges on the PowerPC platform? Is there extra things that
> need to be done from the driver when DMAing on PowerPC (I've looked at
> other drivers and there's nothing obvious). The chip errata doesn't have
> anything on it that looks like it could cause this.
> I'm really hoping this is something that we're doing wrong in the driver
> or the firmware, but we've been through both the firmware and drivers
> countless times and are unable to see anything wrong.
> Any thoughts/ideas would be much appreciated!

Did you actually check the load image for proper alignment?
Like in:

gen2-32 compressed # objdump -x vmlinux.bin
- - - - -
Idx Name Size VMA LMA File off Algn
- - - - -
21 .data.page_aligned 00001800 c068a000 0068a000 0058b000 2**12
22 .data.cacheline_aligned 000026c0 c068b800 0068b800 0058c800 2**6
23 .data.read_mostly 00001e98 c068dec0 0068dec0 0058eec0 2**6

= = = =

I had to make this change to the x86 loader script to get alignment for VIA:

diff --git a/arch/x86/kernel/ b/arch/x86/kernel/
index 62ad500..26f68a5 100644
--- a/arch/x86/kernel/
+++ b/arch/x86/kernel/
@@ -82,7 +82,7 @@ SECTIONS

-  . = ALIGN(32);
   .data.cacheline_aligned : AT(ADDR(.data.cacheline_aligned) - LOAD_OFFSET) {

= = = =

Eyeball your loader script if you haven't already done so.

> Regards,
> Chris

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