linux-kernel - Re: [PATCH v2 2/2] Documentation: dmaengine: Add a documentation for the dma controller API

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Date:	Wed, 22 Oct 2014 23:08:14 +0300
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	Vinod Koul <vinod.koul@...el.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	dmaengine@...r.kernel.org, Russell King <linux@....linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	Antoine Ténart <antoine@...e-electrons.com>,
	Thomas Petazzoni <thomas@...e-electrons.com>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Boris Brezillon <boris@...e-electrons.com>,
	Matt Porter <matt.porter@...aro.org>,
	Gregory Clement <gregory.clement@...e-electrons.com>,
	Nicolas Ferre <nicolas.ferre@...el.com>
Subject: Re: [PATCH v2 2/2] Documentation: dmaengine: Add a documentation for the dma controller API

Hi Maxime,

On Friday 17 October 2014 13:23:56 Maxime Ripard wrote:
> Hi Laurent,
> 
> Just getting back on something...
> 
> On Mon, Oct 06, 2014 at 03:09:48PM +0300, Laurent Pinchart wrote:
> > > +   * device_prep_dma_*
> > > +     - These functions are matching the capabilities you registered
> > > +       previously.
> > > +     - These functions all take the buffer or the scatterlist relevant
> > > +       for the transfer being prepared, and should create a hardware
> > > +       descriptor or a list of descriptors from it
> > > +     - These functions can be called from an interrupt context
> > > +     - Any allocation you might do should be using the GFP_NOWAIT
> > > +       flag, in order not to potentially sleep, but without depleting
> > > +       the emergency pool either.
> > 
> > You could add "Drivers should try to preallocate the data structures they
> > require to prepare a transfer."
> 
> Isn't that obvious?
> 
> I mean, if we're in this function, we're already preparing a
> transfer... And I would expect any programmer that followed CS101 to
> be able to allocate the memory it needs :)

I meant that memory should be pre-allocated earlier (at probe time or channel 
alloc time for instance) to avoid putting pressure on the nowait memory pool.

> The rest of the issues have been fixed, thanks!
> Maxime

-- 
Regards,

Laurent Pinchart

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