linux-kernel - Re: [PATCH V4 1/3] x86: introduce CONFIG_X86_DEV_DMA

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Message-ID: <20120412201533.GA26533@mail.gnudd.com>
Date:	Thu, 12 Apr 2012 22:15:33 +0200
From:	Alessandro Rubini <rubini@...dd.com>
To:	hpa@...or.com
Cc:	mingo@...e.hu, akpm@...ux-foundation.org, jbarnes@...tuousgeek.org,
	fujita.tomonori@....ntt.co.jp, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
	giancarlo.asnaghi@...com, alan@...ux.intel.com
Subject: Re: [PATCH V4 1/3] x86: introduce CONFIG_X86_DEV_DMA_OPS

Peter Anvin:
> I looked over it, and at least this bit seems okay to me.
> 
> Acked-by: H. Peter Anvin <hpa@...or.com>

Thanks.

> I have to express some concern about where this device may be going,
> though.

Well, this CONFIG_X86_DEV_DMA_OPS in particular should not be
concerning, as it's just separating out an x86-64 ifdef so it can be
used and activated by 32-bit systems as well.

As for "this device", it's an I/O Hub that can be used as main chipset
on Atom computers, mainly aimed at the automotive sector. It is a
typical ARM SoC with the usual set of internal peripherals, mounted on
a bridge between PCIe and AMBA, so the PCIe host can access the SoC
peripherals -- and SoC peripherals can DMA to PCIe memory.  Actually,
some SoC devices work well using the existing amba drivers, with
minimal changes that I plan to clean up and submit soon.

The strange DMA requirements here depend on the size limits of the
respective windows: SoC devices can only access a 512MB window of the
host memory, so we need to take care of this with special options.

The current approach is using swiotlb and it works pretty well, though
with a limit of 4MB of swiotlb area.  I've been considering use of the
DMA zone to this aim: internally I have some half-working thing that
resuses the ISA DMA zone for our own aims, raising the DMA limit to
512MB.

Do you think the approach may make sense? I use this basic
thing in Kconfig, to rely on GFP_DMA for the rest:

  config MAX_DMA_PADDR
          int
          default 536870912 if MAX_DMA_PADDR_512M
          default 16777216

  config MAX_DMA_PADDR_512M
          bool

(actually, we'd benefit from being able to use hex in defaults)

Is this worth exploring, to possibly submit a patch in this direction?

thanks
/alessandro
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