linux-kernel - Re: [PATCH] pci-dma: Fix x86 dma_alloc_coherent to fully clear all pages returned

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Message-ID: <1422655318.9530.15.camel@schen9-desk2.jf.intel.com>
Date:	Fri, 30 Jan 2015 14:01:58 -0800
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Akinobu Mita <akinobu.mita@...il.com>,
	Mathias Nyman <mathias.nyman@...ux.intel.com>,
	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-usb@...r.kernel.org
Subject: Re: [PATCH] pci-dma: Fix x86 dma_alloc_coherent to fully clear all
 pages returned

On Fri, 2015-01-30 at 11:58 -0800, Greg KH wrote:

> 
> Shouldn't this go to stable trees too?
> 

Yes. Added the stable tag in updated patch that's resent.

> Also, why is the xhci driver not asking for the memory it is going to
> need?  If it wants to use the full page, shouldn't it ask for it?
> 

I agree that xhci should have done that, but it didn't.  Commit 
d92ef66c4f8f ("x86: make dma_alloc_coherent() return zeroed memory 
if CMA is enabled") changed the behavior of dma_alloc_coherent
by clearing only the memory being asked for.

So for backward compatibility, clearing the pages 
completely to revert to dma_alloc_coherent's original
behavior is probably the safe thing to do.

Thanks.

Tim

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