linux-kernel - Re: [PATCH] dma-direct: Fix return value of dma_direct

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Date:   Thu, 13 Dec 2018 20:58:31 +0100
From:   Christoph Hellwig <hch@....de>
To:     "Lendacky, Thomas" <Thomas.Lendacky@....com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        "alexander.h.duyck@...ux.intel.com" 
        <alexander.h.duyck@...ux.intel.com>,
        "open list:INTEL IOMMU (VT-d)" <iommu@...ts.linux-foundation.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] dma-direct: Fix return value of dma_direct_supported

On Thu, Dec 13, 2018 at 07:45:57PM +0000, Lendacky, Thomas wrote:
> So I think this needs to be __phys_to_dma() here. I only recently got a
> system that had a device where the driver only supported 32-bit DMA and
> found that when SME is active this returns 0 and causes the driver to fail
> to initialize. This is because the SME encryption bit (bit 47) is part of
> the check when using phys_to_dma(). During actual DMA when SME is active,
> bounce buffers will be used for anything that can't meet the 48-bit
> requirement. But for this test, using __phys_to_dma() should give the
> desired results, right?
> 
> If you agree with this, I'll submit a patch to make the change. I missed
> this in 4.19, so I'll need to submit something to stable, too. The only
> issue there is the 4.20 fix won't apply cleanly to 4.19.

Yes, please send a patch.  Please make sure it includes a code comment
that explains why the __-prefixed version is used.