DMA Mapping Error in ppc64

Oliver oohall at
Thu Mar 22 15:54:15 AEDT 2018

On Thu, Mar 22, 2018 at 8:00 AM, Jared Bents
<jared.bents at> wrote:
> Hi all,
> Apologies for the amount of information but we've been debugging this
> for a while and I wanted to get what we are seeing captured as much as
> possible.  We are a T1042 processor and have a total 8GB DDR and our
> kernel version is fsl-sdk-v2.0-1703 (linux v4.1.35) as that is the
> latest version supplied by NXP.
> A while ago we ported from 32 bit to 64 bit.  Everything continued to
> work except the ath10k module we have.  So as a first step, we checked
> to see if an ath9k module also failed to work and it was also no
> longer working.  The ath10k is working fine on a 32 bit system but
> it's not working on 64 bit system as we are getting dma mapping errors
> when trying to initialize the wifi modules.
> pci_bus 0002:01: bus scan returning with max=01
> pci_bus 0002:01: busn_res: [bus 01] end is updated to 01
> pci_bus 0002:00: bus scan returning with max=01
> ath10k_pci 0000:01:00.0: unable to get target info from device
> ath10k_pci 0000:01:00.0: could not get target info (-5)
> ath10k_pci 0000:01:00.0: could not probe fw (-5)
> ath10k_pci 0001:01:00.0: Direct firmware load for
> ath10k/cal-pci-0001:01:00.0.bin failed with error -2
> First, we have tried the mainline kernel (v4.15)  to see if that would
> fix the issue, it did not.  So I made a patch for the ath10k driver to
> restrict to just GFP_DMA areas when allocating memory or creating
> sk_buffs and have attached it.  The ath10k wifi modules now initialize
> correctly but when I try to connect them and send traffic, they get a
> DMA mapping error from the sk_buff that it receives from elsewhere in
> the kernel.  So while the driver appears to be fixable with the patch,
> the modules are still unusable due to data being sent to the driver
> when ath10k_tx is called and it tries to dma map with the provided
> skb.  Also, according to the ath10k mailing list, GFP_DMA is not
> supposed to be used in general.  The error below is the same sort of
> dma mapping error that is seen when initializing the modules without
> the patch to OR with GFP_DMA.
> ath10k_pci 0001:01:00.0: failed to transmit packet, dropping: -5
> We asked on the ath10k mailing list if anyone else is having this
> problem and no one else seems to have the issue but they are using
> different architectures (ARM or X86). As a result, it does not seem to
> be a driver issue to us but something within the PowerPC arch.  So we
> dug a little deeper to try to find what addresses being mapped are
> working and what address being mapped are not working.
> We found that when the virtual address of data pointer (a member of
> sk_buff) is above ~3.7 GB RAM address range then return address from
> dma_map_single API is failed to validate in dma_mapping_error
> function.
> We also noticed that in a 64bit machine sometimes ping is working and
> because of the virtual address is under ~3.7GAM RAM address range.  So
> if we set mem=2048M in the bootargs, the ath10k module works
> perfectly, however this isn't a real solution since it cuts our
> available RAM from 8GB to 2GB.

I think there's a known issue with the freescale PCIe root complex
where it can't DMA beyond the 4GB mark. There's a workaround in
the form of limit_zone_pfn() which you can use to put the lower 4GB into
ZONE_DMA32 and allocate from there rather than ZONE_NORMAL.
For details of how to use it have a look at corenet_gen_setup_arch() in

Hope that helps,

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