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

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Date:	Wed, 07 Apr 2010 17:59:26 -0600
From:	Robert Hancock <hancockrwd@...il.com>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	Takashi Iwai <tiwai@...e.de>, Daniel Mack <daniel@...aq.de>,
	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 04/07/2010 01:13 PM, 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.

The comment is wrong (or at least outdated or based on an incorrect 
assumption), but you're right, currently 64-bit DMA is not used on any 
EHCI controllers. It could be, but it sounded like the consensus was it 
wasn't worth the risk. Apparently Windows 7 started using it, and then 
had to put out a patch because some NVidia EHCI controllers indicated 
64-bit support but it didn't work properly. So you'd have to blacklist 
those controllers, at least.
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