linux-kernel - Re: USB transfer_buffer allocations on 64bit systems

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Message-ID: <20100407175935.GN30801@buzzloop.caiaq.de>
Date:	Wed, 7 Apr 2010 19:59:35 +0200
From:	Daniel Mack <daniel@...aq.de>
To:	Takashi Iwai <tiwai@...e.de>
Cc:	Alan Stern <stern@...land.harvard.edu>, Greg KH <greg@...ah.com>,
	linux-kernel@...r.kernel.org, Pedro Ribeiro <pedrib@...il.com>,
	akpm@...ux-foundation.org, Greg KH <gregkh@...e.de>,
	alsa-devel@...a-project.org, linux-usb@...r.kernel.org
Subject: Re: USB transfer_buffer allocations on 64bit systems

On Wed, Apr 07, 2010 at 07:55:20PM +0200, Takashi Iwai wrote:
> At Wed, 7 Apr 2010 18:16:03 +0200,
> Daniel Mack wrote:
> > 
> > On Wed, Apr 07, 2010 at 11:55:19AM -0400, Alan Stern wrote:
> > > On Wed, 7 Apr 2010, Greg KH wrote:
> > > 
> > > > Alan, any objection to just using usb_buffer_alloc() for every driver?
> > > > Or is that too much overhead?
> > > 
> > > I don't know what the overhead is.  But usb_buffer_alloc() requires the 
> > > caller to keep track of the buffer's DMA address, so it's not a simple 
> > > plug-in replacement.  In addition, the consistent memory that 
> > > usb_buffer_alloc() provides is a scarce resource on some platforms.
> > > 
> > > Writing new functions is the way to go.
> > 
> > Ok, I'll write some dummies for usb_malloc() and usb_zalloc() which
> > will just call kmalloc() with GFP_DMA32 for now.
> 
> Can't we provide only zalloc() variant?  Zero'ing doesn't cost much,
> and the buffer allocation shouldn't be called too often.
> 
> > And while at it,
> > usb_alloc_buffer() will be renamed to usb_alloc_consistent().
> 
> Most of recent functions are named with "coherent".

I agree to both points, will do so unless anyone has a harsh opinion
about that.

Another thing: I guess we don't need a corresponding free() function
that just calls kfree(), right? Or should we introduce it now to be
flexible for future extensions?

Daniel

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