linux-kernel - Re: [PATCH 5/5] dma-direct: always allow dma mask <= physiscal memory size

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Message-ID: <8402ecc9c8ed9c69ad3e91eca4d07a5ab077b22d.camel@kernel.crashing.org>
Date:   Tue, 09 Oct 2018 16:01:18 +1100
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Alexander Duyck <alexander.duyck@...il.com>,
        Christoph Hellwig <hch@....de>,
        Guenter Roeck <linux@...ck-us.net>
Cc:     "open list:INTEL IOMMU (VT-d)" <iommu@...ts.linux-foundation.org>,
        Robin Murphy <robin.murphy@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 5/5] dma-direct: always allow dma mask <= physiscal
 memory size

On Wed, 2018-10-03 at 16:10 -0700, Alexander Duyck wrote:
> > -        * Because 32-bit DMA masks are so common we expect every architecture
> > -        * to be able to satisfy them - either by not supporting more physical
> > -        * memory, or by providing a ZONE_DMA32.  If neither is the case, the
> > -        * architecture needs to use an IOMMU instead of the direct mapping.
> > -        */
> > -       if (mask < phys_to_dma(dev, DMA_BIT_MASK(32)))
> > +       u64 min_mask;
> > +
> > +       if (IS_ENABLED(CONFIG_ZONE_DMA))
> > +               min_mask = DMA_BIT_MASK(ARCH_ZONE_DMA_BITS);
> > +       else
> > +               min_mask = DMA_BIT_MASK(32);
> > +
> > +       min_mask = min_t(u64, min_mask, (max_pfn - 1) << PAGE_SHIFT);
> > +
> > +       if (mask >= phys_to_dma(dev, min_mask))
> >                  return 0;
> > -#endif
> >          return 1;
> >   }
> 
> So I believe I have run into the same issue that Guenter reported. On
> an x86_64 system w/ Intel IOMMU. I wasn't able to complete boot and
> all probe attempts for various devices were failing with -EIO errors.
> 
> I believe the last mask check should be "if (mask < phys_to_dma(dev,
> min_mask))" not a ">=" check.

Right, that test is backwards. I needed to change it here too (powermac
with the rest of the powerpc series).

Cheers,
Ben.