linux-kernel - Re: [PATCH v2 8/8] habanalabs: enable 64-bit DMA mask in POWER9

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Date:   Tue, 11 Jun 2019 23:53:14 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Oliver O'Halloran <oohall@...il.com>
Cc:     Oded Gabbay <oded.gabbay@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>, linuxppc-dev@...abs.org,
        Christoph Hellwig <hch@...radead.org>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 8/8] habanalabs: enable 64-bit DMA mask in POWER9

On Wed, Jun 12, 2019 at 04:35:22PM +1000, Oliver O'Halloran wrote:
> Setting a 48 bit DMA mask doesn't work today because we only allocate
> IOMMU tables to cover the 0..2GB range of PCI bus addresses.

I don't think that is true upstream, and if it is we need to fix bug
in the powerpc code.  powerpc should be falling back treating a 48-bit
dma mask like a 32-bit one at least, that is use dynamic iommu mappings
instead of using the direct mapping.  And from my reding of 
arch/powerpc/kernel/dma-iommu.c that is exactly what it does.