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

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Message-ID: <1422656187.9530.17.camel@schen9-desk2.jf.intel.com>
Date:	Fri, 30 Jan 2015 14:16:27 -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 14:07 -0800, Greg KH wrote:
> On Fri, Jan 30, 2015 at 02:01:58PM -0800, Tim Chen wrote:
> > 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.
> 
> We don't "need" any backward compatility, why not fix the broken drivers
> that are using memory outside of what they are asking for?  That's not
> ok no matter what, right?

Not disagreeing with you.  I'll be equally happy if the xhci folks
can fix the driver.  Mathias?

Tim

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