linux-kernel - Re: [RFC] Staging: IIO: New ABI V2

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Message-ID: <201002161418.28097.rgetz@blackfin.uclinux.org>
Date:	Tue, 16 Feb 2010 14:18:27 -0500
From:	Robin Getz <rgetz@...ckfin.uclinux.org>
To:	Jonathan Cameron <jic23@....ac.uk>
CC:	Greg KH <gregkh@...e.de>, Mike Frysinger <vapier.adi@...il.com>,
	Kay Sievers <kay.sievers@...y.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dmitry Torokhov <dtor@...l.ru>,
	"Hennerich, Michael" <Michael.Hennerich@...log.com>,
	Manuel Stahl <manuel.stahl@....fraunhofer.de>,
	Robin Getz <rgetz@...ckfin.uclinux.org>,
	"Trisal, Kalhan" <kalhan.trisal@...el.com>,
	"Zhang, Xing Z" <xing.z.zhang@...el.com>,
	Ira Snyder <iws@...o.caltech.edu>,
	Jean Delvare <khali@...ux-fr.org>,
	Samu Onkalo <samu.p.onkalo@...ia.com>,
	Stefani Seibold <stefani@...bold.net>
Subject: Re: [RFC] Staging: IIO: New ABI V2

On Tue 16 Feb 2010 06:03, Jonathan Cameron pondered:
> On 02/16/10 02:49, Greg KH wrote:
> > On Mon, Feb 15, 2010 at 07:58:12PM -0500, Mike Frysinger wrote:
> >> On Mon, Feb 15, 2010 at 15:26, Robin Getz wrote:
> >>> [snip]
> >>> What exists today still requires a copy_[to|from]_user when using
> >>> the ring buffer (and then another cache_flush if you are dma'ing
> >>> things). These seems pretty expensive and will consume extra cycles
> >>> that will limit throughput. 
> >>>
> >>> Any thoughts to a mmaped interface directly to the IIO ring buffer, 
> >>> so the system could avoid some of the above overhead? (This is what
> >>> we had to do for some other drivers - which were able to handle a 40
> >>> MSample/second data processed by userspace for a soft radio).
> >>
> >> does sysfs currently support mmap-ing of files in there ?
> > 
> > For binary files, yes.  If you are going to use mmap, use a character
> > device node instead please, that's not what sysfs is for.
> All the buffer access is done via character device nodes anyway.
> 
> For anyone entering the discussion at this point:
> Only really simple IIO drivers (for typically very slow devices)
> are principally accessed through sysfs.  For these fast devices we
> probably wouldn't provide that route at all, merely using sysfs to
> describe the parameters of the device and buffer being used.

Can we be a little more specific - what in your mind is "very slow"? 
and "fast"?

Is it designated by samples per second? (and bits per sample doesn't matter?) 
or is it the result (bits per sample * samples per second/8 == bytes/second?)

Many people I know would call a 1Mega sample per second converter very slow, 
but the kernel handling a memcpy of a continuous 2Mbyte/second (16-bits per 
sample), stream seems a little wasteful.

Thanks
-Robin
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