linux-kernel - Re: [PATCH] usb: chipidea: Configure DMA properties and ops from DT

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Message-ID: <20160318105655.GE19428@n2100.arm.linux.org.uk>
Date:	Fri, 18 Mar 2016 10:56:55 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Peter Chen <hzpeterchen@...il.com>
Cc:	Arnd Bergmann <arnd@...db.de>, Peter Chen <Peter.Chen@....com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Rajesh Bhagat <rajesh.bhagat@....com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Bjorn Andersson <bjorn.andersson@...aro.org>,
	Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
	linux-arm-msm <linux-arm-msm@...r.kernel.org>,
	Li Yang <leoli@...escale.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] usb: chipidea: Configure DMA properties and ops from DT

On Fri, Mar 18, 2016 at 09:54:14AM +0800, Peter Chen wrote:
> Although I don't know what kinds of bugs it may have, it may be
> met before, otherwise, why most of platform drivers need to call
> dma_set_coherent_mask or dma_coerce_mask_and_coherent explicitly

See Documentation/DMA-API.txt, specifically the section starting

  Part Ic - DMA addressing limitations
  ------------------------------------

and also Documentation/DMA-API-HOWTO.txt, the section on

  DMA addressing limitations

which provides further information.

Drivers using DMA should be using dma_set_mask_and_coherent() _or_
one of dma_set_mask() and dma_set_coherent_mask() depending on which
types of DMA they wish to perform.  Drivers should not use
dma_coerce_mask_and_coherent() except in exceptional circumstances:
that function is more a marker that they or some bus/platform code
is doing something wrong.

-- 
RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
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