linux-kernel - Re: [PATCH v2 1/2] mfd: viperboard: allocate I/O buffer separately

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Message-ID: <20140924122658.GD17019@localhost>
Date:	Wed, 24 Sep 2014 14:26:58 +0200
From:	Johan Hovold <johan@...nel.org>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	Johan Hovold <johan@...nel.org>,
	Octavian Purdila <octavian.purdila@...el.com>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mfd: viperboard: allocate I/O buffer separately

On Wed, Sep 24, 2014 at 01:00:02PM +0100, Lee Jones wrote:
> On Wed, 24 Sep 2014, Johan Hovold wrote:
> 
> > On Wed, Sep 24, 2014 at 11:12:06AM +0100, Lee Jones wrote:
> > > On Mon, 22 Sep 2014, Octavian Purdila wrote:
> > > 
> > > > Currently the I/O buffer is allocated part of the device status
> > > > structure, potentially sharing the same cache line with other members
> > > > in this structure.
> > > > 
> > > > Allocate the buffer separately, to avoid the I/O operations corrupting
> > > > the device status structure due to cache line sharing.
> > > > 
> > > > Compiled tested only as I don't have access to hardware.
> > > > 
> > > > Signed-off-by: Octavian Purdila <octavian.purdila@...el.com>
> > > > ---
> > > >  drivers/mfd/viperboard.c       | 8 ++++++++
> > > >  include/linux/mfd/viperboard.h | 2 +-
> > > >  2 files changed, 9 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/mfd/viperboard.c b/drivers/mfd/viperboard.c
> > > > index e00f534..5f62f4e 100644
> > > > --- a/drivers/mfd/viperboard.c
> > > > +++ b/drivers/mfd/viperboard.c
> > > > @@ -64,6 +64,12 @@ static int vprbrd_probe(struct usb_interface *interface,
> > > >  		return -ENOMEM;
> > > >  	}
> > > >  
> > > > +	vb->buf = kzalloc(sizeof(struct vprbrd_i2c_write_msg), GFP_KERNEL);
> > > 
> > > Can you obtain the 'struct device' first then use managed resources
> > > (devm_*)?
> > 
> > I think any devres conversion should be done in a follow-up patch and
> > not be included in the fix (e.g. in order to facilitate backporting). We
> > also don't want to mix allocation schemes.
> 
> I agree, but equally I'm not keen on accepting this patch as I believe
> it could be done better.
>
> Please submit two patches, one converting to shared resources and this
> being the subsequent one, fixed up to do the right thing.

A buffer-corruption fix is a candidate for stable, whereas a devres
conversion (clean up) is not. Hence the former should not depend on the
latter.

Johan
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