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Subject: Re: [PATCH v2 8/8] habanalabs: enable 64-bit DMA mask in POWER9
From:   Benjamin Herrenschmidt <benh@kernel.crashing.org>
To:     "Oliver O'Halloran" <oohall@gmail.com>
Cc:     Oded Gabbay <oded.gabbay@gmail.com>,
        Greg KH <gregkh@linuxfoundation.org>, linuxppc-dev@ozlabs.org,
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        Russell Currey <ruscur@au1.ibm.com>,
        "Linux-Kernel@Vger. Kernel. Org" <linux-kernel@vger.kernel.org>
Date:   Wed, 12 Jun 2019 18:17:46 +1000
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On Wed, 2019-06-12 at 15:45 +1000, Oliver O'Halloran wrote:
> 
> Also, are you sure about the MSI thing? The IODA3 spec says the only
> important bits for a 64bit MSI are bits 61:60 (to hit the window) and
> the lower bits that determine what IVE to use. Everything in between
> is ignored so ORing in bit 59 shouldn't break anything.

On IODA3... could be different on another system. My point is you can't
just have a fixed setting for all top bits for DMA & MSIs.

> > This will only work as long as all of the system memory can be
> > addressed at an offset from that fixed address that itself fits your
> > device addressing capabilities (50 bits in this case). It may or may
> > not be the case but there's no way to check since the DMA mask logic
> > won't really apply.
> > 
> > You might want to consider fixing your HW in the next iteration... This
> > is going to bite you when x86 increases the max physical memory for
> > example, or on other architectures.
> 
> Yes, do this. The easiest way to avoid this sort of wierd hack is to
> just design the PCIe interface to the spec in the first place.

Ben.