From: Christoph Hellwig <hch@lst.de>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Nikita Yushchenko <nikita.yoush@cogentembedded.com>,
Will Deacon <will.deacon@arm.com>, Arnd Bergmann <arnd@arndb.de>,
linux-arm-kernel@lists.infradead.org,
Catalin Marinas <catalin.marinas@arm.com>,
linux-kernel@vger.kernel.org, linux-renesas-soc@vger.kernel.org,
Simon Horman <horms@verge.net.au>,
Bjorn Helgaas <bhelgaas@google.com>,
artemi.ivanov@cogentembedded.com, fkan@apm.com,
Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v2] arm64: do not set dma masks that device connection can't handle
Date: Tue, 10 Jan 2017 15:57:16 +0100 [thread overview]
Message-ID: <20170110145716.GD27156@lst.de> (raw)
In-Reply-To: <11daacde-5399-039f-80a3-01d7bd13e9e8@arm.com>
On Tue, Jan 10, 2017 at 01:25:12PM +0000, Robin Murphy wrote:
> We still need a way for drivers to communicate a device's probed
> addressing capability to SWIOTLB, so there's always going to have to be
> *some* sort of public interface. Personally, the change in semantics I'd
> like to see is to make dma_set_mask() only fail if DMA is entirely
> disallowed - in the normal case it would always succeed, but the DMA API
> implementation would be permitted to set a smaller mask than requested
> (this is effectively what the x86 IOMMU ops do already).
Yes, this sounds reasonable.
> The significant
> work that would require, though, is changing all the drivers currently
> using this sort of pattern:
>
> if (!dma_set_mask(dev, DMA_BIT_MASK(64))
> /* put device into 64-bit mode */
> else if (!dma_set_mask(dev, DMA_BIT_MASK(32))
> /* put device into 32-bit mode */
> else
> /* error */
While we have this pattern in a lot of places it's already rather
pointless on most architectures as the first dma_set_mask call
won't ever fail for the most common dma_ops implementations.
> to something like this:
>
> if (!dma_set_mask(dev, DMA_BIT_MASK(64))
> /* error */
> if (dma_get_mask(dev) > DMA_BIT_MASK(32))
> /* put device into 64-bit mode */
> else
> /* put device into 32-bit mode */
>
> Which would be a pretty major job.
I don't think it's too bad. Also for many modern devices there is no
need to put the device into a specific mode. It's mostly a historic
issue from the PCI/PCI-X days with the less efficient DAC addressing
scheme.
next prev parent reply other threads:[~2017-01-10 14:57 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-09 7:30 [PATCH v2] arm64: do not set dma masks that device connection can't handle Nikita Yushchenko
2017-01-10 11:51 ` Will Deacon
2017-01-10 12:47 ` Nikita Yushchenko
2017-01-10 13:12 ` Arnd Bergmann
2017-01-10 13:25 ` Robin Murphy
2017-01-10 13:42 ` Arnd Bergmann
2017-01-10 14:16 ` Robin Murphy
2017-01-10 15:06 ` Arnd Bergmann
2017-01-11 12:37 ` Nikita Yushchenko
2017-01-11 16:21 ` Arnd Bergmann
2017-01-11 18:28 ` Robin Murphy
2017-01-10 14:59 ` Christoph Hellwig
2017-01-10 14:01 ` Nikita Yushchenko
2017-01-10 14:57 ` Christoph Hellwig [this message]
2017-01-10 14:51 ` Christoph Hellwig
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