From: Bjorn Helgaas <bhelgaas@google.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: linux-doc@vger.kernel.org,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Joerg Roedel <joro@8bytes.org>,
Randy Dunlap <rdunlap@infradead.org>,
Liviu Dudau <Liviu.Dudau@arm.com>,
linux-kernel@vger.kernel.org,
James Bottomley <jbottomley@parallels.com>,
linux-pci@vger.kernel.org, David Woodhouse <dwmw2@infradead.org>
Subject: Re: [PATCH v2 4/5] iommu: Use dma_addr_t for IOVA arguments
Date: Wed, 7 May 2014 18:18:14 -0600 [thread overview]
Message-ID: <20140508001814.GB4951@google.com> (raw)
In-Reply-To: <45161364.3Gyo1xJJLV@wuerfel>
On Wed, May 07, 2014 at 09:58:58AM +0200, Arnd Bergmann wrote:
> On Tuesday 06 May 2014 16:48:40 Bjorn Helgaas wrote:
> > Convert the "iova" arguments of iommu_map(), iommu_unmap(), etc., from
> > "unsigned long" to dma_addr_t.
> >
> > bb5547acfcd8 ("iommu/fsl: Make iova dma_addr_t in the iommu_iova_to_phys
> > API") did this for iommu_iova_to_phys(), but didn't fix the rest of the
> > IOMMU API.
>
> This patch looks 100% correct, but I'm not convinced it's a good idea:
> On 32-bit platforms (i.e. most of the ones you change), doing 64-bit
> arithmetic has a noticeable overhead. I am not aware of an IOMMU that
> actually uses 64-bit DMA addresses on its slave side, usually they
> are used to translate addresses from 32-bit masters into 64-bit
> memory addresses, so using 'unsigned long' seems better from a practical
> point of view as opposed to the strict type correctness.
The current x86 IOMMUs support DMA addresses larger than 32 bits, but
obviously those platforms usually run 64-bit kernels so "unsigned
long" is already 64 bits.
I guess you're thinking about cases where "unsigned long" is 32 bits,
the IOMMU only supports 32 bit DMA addresses, and dma_addr_t is 64
bits. If the IOMMU only supports 32-bit DMA addresses, is there any
value in having a 64-bit dma_addr_t?
Bjorn
next prev parent reply other threads:[~2014-05-08 0:18 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-06 22:48 [PATCH v2 0/5] Clean up DMA API & IOMMU dma_addr_t usage Bjorn Helgaas
2014-05-06 22:48 ` [PATCH v2 1/5] DMA-API: Clarify physical/bus address distinction Bjorn Helgaas
2014-05-07 7:37 ` Arnd Bergmann
2014-05-07 18:43 ` Bjorn Helgaas
2014-05-08 9:24 ` Greg Kroah-Hartman
2014-05-06 22:48 ` [PATCH v2 2/5] DMA-API: Change dma_declare_coherent_memory() CPU address to phys_addr_t Bjorn Helgaas
2014-05-07 7:38 ` Arnd Bergmann
2014-05-06 22:48 ` [PATCH v2 3/5] sh/PCI: Pass GAPSPCI_DMA_BASE CPU address to dma_declare_coherent_memory() Bjorn Helgaas
2014-05-07 7:55 ` Arnd Bergmann
2014-05-07 8:15 ` Arnd Bergmann
2014-05-07 23:18 ` Bjorn Helgaas
2014-05-08 11:36 ` Arnd Bergmann
2014-05-06 22:48 ` [PATCH v2 4/5] iommu: Use dma_addr_t for IOVA arguments Bjorn Helgaas
2014-05-07 7:58 ` Arnd Bergmann
2014-05-08 0:18 ` Bjorn Helgaas [this message]
2014-05-08 10:44 ` Arnd Bergmann
2014-05-08 20:30 ` Bjorn Helgaas
2014-05-09 9:58 ` David Woodhouse
2014-05-09 15:32 ` Bjorn Helgaas
2014-05-09 19:52 ` Arnd Bergmann
2014-05-09 20:19 ` Bjorn Helgaas
2014-05-09 20:25 ` James Bottomley
2014-05-06 22:48 ` [PATCH v2 5/5] iommu/exynos: Remove unnecessary "&" from function pointers Bjorn Helgaas
2014-05-07 7:59 ` Arnd Bergmann
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