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

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Date:	Wed, 7 Apr 2010 17:33:13 -0700
From:	Greg KH <greg@...ah.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Takashi Iwai <tiwai@...e.de>, Daniel Mack <daniel@...aq.de>,
	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 03:13:11PM -0400, Alan Stern wrote:
> On Wed, 7 Apr 2010, Takashi Iwai wrote:
> 
> > > 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.
> 
> Linus specifically requested us to avoid using kzalloc in usbfs.  I
> can't find the message in the email archives, but Greg KH should be
> able to confirm it.
> 
> As long as we're imitating kmalloc for one use, we might as well make
> it available to all.
> 
> > > And while at it,
> > > usb_alloc_buffer() will be renamed to usb_alloc_consistent().
> > 
> > Most of recent functions are named with "coherent".
> 
> Yes, the terminology got a little confused between the PCI and DMA
> realms.  I agree, "coherent" is better.
> 
> BTW, although some EHCI controllers may support 64-bit DMA, the driver 
> contains this:
> 
> 	if (HCC_64BIT_ADDR(hcc_params)) {
> 		ehci_writel(ehci, 0, &ehci->regs->segment);
> #if 0
> // this is deeply broken on almost all architectures
> 		if (!dma_set_mask(hcd->self.controller, DMA_BIT_MASK(64)))
> 			ehci_info(ehci, "enabled 64bit DMA\n");
> #endif
> 	}
> 
> I don't know if the comment is still true, but until the "#if 0" is 
> removed, ehci-hcd won't make use of 64-bit DMA.

I think someone tried to remove it recently, but I wouldn't let them :)

What a mess, hopefully xhci will just take over and save the world from
this whole thing...

thanks,

greg k-h
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