linux-kernel - Re: [PATCH 6/7] MIPS: SGI-IP27: use generic PCI driver

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Date:   Wed, 20 Feb 2019 16:20:53 +0100
From:   Thomas Bogendoerfer <tbogendoerfer@...e.de>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        James Hogan <jhogan@...nel.org>, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-pci@...r.kernel.org,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Paul Burton <paul.burton@...s.com>,
        Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [PATCH 6/7] MIPS: SGI-IP27: use generic PCI driver

On Wed, 20 Feb 2019 07:10:56 -0800
Christoph Hellwig <hch@...radead.org> wrote:

> On Mon, Feb 18, 2019 at 11:58:07AM +0100, Thomas Bogendoerfer wrote:
> > This of course will break SH7786. To fix both cases how about making dma_pfn_offset
> >
> > a signed long ?
> 
> Yes, making it signed sounds like a good idea.

but at least my implementation looks a little bit ugly:

diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
index b7338702592a..b72b1cba8911 100644
--- a/include/linux/dma-direct.h
+++ b/include/linux/dma-direct.h
@@ -12,14 +12,20 @@ static inline dma_addr_t __phys_to_dma(struct device *dev, phys_addr_t paddr)
 {
        dma_addr_t dev_addr = (dma_addr_t)paddr;
 
-       return dev_addr - ((dma_addr_t)dev->dma_pfn_offset << PAGE_SHIFT);
+       if (dev->dma_pfn_offset > 0)
+               return dev_addr + ((dma_addr_t)dev->dma_pfn_offset << PAGE_SHIFT);
+
+       return dev_addr - ((dma_addr_t)-dev->dma_pfn_offset << PAGE_SHIFT);
 }
 
 static inline phys_addr_t __dma_to_phys(struct device *dev, dma_addr_t dev_addr)
 {
        phys_addr_t paddr = (phys_addr_t)dev_addr;
 
-       return paddr + ((phys_addr_t)dev->dma_pfn_offset << PAGE_SHIFT);
+       if (dev->dma_pfn_offset > 0)
+               return paddr - ((phys_addr_t)dev->dma_pfn_offset << PAGE_SHIFT);
+
+       return paddr + ((phys_addr_t)-dev->dma_pfn_offset << PAGE_SHIFT);
 }
 
 static inline bool dma_capable(struct device *dev, dma_addr_t addr, size_t size)



For the v2 version of the IP27 rework I've used the mach-XXX include structure
of the MIPS tree to implement __phy_to_dma/__dma_to_phys for IP27. I'd prefer
it that way.

Thomas.

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