netdev - Re: [PATCH] stmmac: explicitly zero des0 & des1 on init

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Date:	Tue, 23 Jun 2015 15:04:31 +0000
From:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:	"davem@...emloft.net" <davem@...emloft.net>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"arc-linux-dev@...opsys.com" <arc-linux-dev@...opsys.com>,
	"peppe.cavallaro@...com" <peppe.cavallaro@...com>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] stmmac: explicitly zero des0 & des1 on init

Hi David,

On Mon, 2015-06-22 at 09:43 +0300, Alexey Brodkin wrote:
> Hi David,
> 
> On Sun, 2015-06-21 at 09:29 -0700, David Miller wrote:
> > From: Alexey Brodkin <Alexey.Brodkin@...opsys.com>
> > Date: Tue, 16 Jun 2015 20:40:41 +0300
> > 
> > > Current implementtion of descriptor init procedure only takes 
> > > care 
> > > about
> > > ownership flag. While it is perfectly possible to have underlying 
> > > 
> > > memory
> > > filled with garbage on boot or driver installation.
> > > 
> > > And randomly set flags in non-zeroed des0 and des1 fields may 
> > > lead 
> > > to
> > > unpredictable behavior of the GMAC DMA block.
> > > 
> > > Solution to this problem is as simple as explicit zeroing of both 
> > > 
> > > des0
> > > and des1 fields of all buffer descriptors.
> > > 
> > > Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>
> > 
> > If you need the memory zero initialized, use dma_zalloc_coherent().
> 
> Indeed usage of dma_zalloc_coherent() will resolve observed issue.
> But since buffer descriptors are reused extensively I would say that
> explicit zeroing of fields with flags is useful. Probably I need to 
> add
> this clarification in commit message.
> 
> And then if we do that explicit zeroing of flags and other fields 
> which
> hold data size and addresses of data buffer and the next descriptor 
> in
> chain are all get set later we may not care about allocation of 
> zeroed
> memory.

I'm wondering if my comment makes sense and should I just change commit
message or you'd prefer to still use dma_zalloc_coherent() during
driver probe?

-Alexey--
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