linux-kernel - Re: [PATCHv3 08/16] staging: vme_user: provide DMA functionality

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Message-ID: <559D2879.30602@ge.com>
Date:	Wed, 8 Jul 2015 14:41:13 +0100
From:	Martyn Welch <martyn.welch@...com>
To:	Alessio Igor Bogani <alessioigorbogani@...il.com>,
	Dmitry Kalinkin <dmitry.kalinkin@...il.com>
CC:	<devel@...verdev.osuosl.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Igor Alekseev <igor.alekseev@...p.ru>,
	LKML <linux-kernel@...r.kernel.org>,
	Manohar Vanga <manohar.vanga@...il.com>
Subject: Re: [PATCHv3 08/16] staging: vme_user: provide DMA functionality

On 07/07/15 13:51, Alessio Igor Bogani wrote:
>> Current VME stack links windows not to the boards, but to device drivers.
>> >Driver
>> >could potentially minimise window usage within it’s scope (any sort of
>> >window
>> >reusing, like mapping whole A16 once to be used with all boards), but this
>> >won’t
>> >work across multiple drivers. Even if all of your drivers are window-wise
>> >economic,
>> >they will still need some amount of windows per each driver. Not that we
>> >have that
>> >many kernel drivers...
> Yes you can share a window/image between all boards of the same type
> (in effect we are porting our drivers in this way)*but*  it isn't the
> expected way to work (see Documentation/vme_api.txt struct
> vme_driver's probe() and match() functions and the GE PIO2 VME
> driver).

I think it's perfectly valid to use a single window to dynamically map 
to the address space belonging to one of a number of devices supported 
by a single driver. I think this is almost preferable to mapping a large 
window over a large portion of the VME address space to drive a number 
of devices as (depending on there spacing in the VME address space) the 
latter could cause issues with filling available PCI address space. 
Admittedly this is more of a problem on 32-bit systems, but...

-- 
Martyn Welch (Lead Software Engineer)  | Registered in England and Wales
GE Intelligent Platforms               | (3828642) at 100 Barbirolli Square
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