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

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Date:	Thu, 08 Apr 2010 18:01:27 -0600
From:	Robert Hancock <hancockrwd@...il.com>
To:	Greg KH <greg@...ah.com>
CC:	Alan Stern <stern@...land.harvard.edu>,
	alsa-devel@...a-project.org, linux-usb@...r.kernel.org,
	Takashi Iwai <tiwai@...e.de>, Greg KH <gregkh@...e.de>,
	linux-kernel@...r.kernel.org, Pedro Ribeiro <pedrib@...il.com>,
	akpm@...ux-foundation.org, sarah.a.sharp@...ux.intel.com
Subject: Re: USB transfer_buffer allocations on 64bit systems

On 04/07/2010 06:33 PM, Greg KH wrote:
> 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...

True.. except for the fact that the xhci driver currently doesn't do 
64-bit DMA either, nor does it support MSI even though the HW supports 
it (surprisingly enough the NEC Windows driver does, MSI-X even). At 
this point only Intel likely knows how to do this properly, though, 
since AFAICS the spec isn't publicly available yet.
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