linux-kernel - Re: [PATCH] mfd: inherit coherent_dma

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Message-ID: <20140923071352.65a6e9ec@bbrezillon>
Date:	Tue, 23 Sep 2014 07:13:52 +0200
From:	Boris BREZILLON <boris.brezillon@...e-electrons.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Samuel Ortiz <sameo@...ux.intel.com>,
	Lee Jones <lee.jones@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mfd: inherit coherent_dma_mask from parent device

Hi Arnd,

On Mon, 22 Sep 2014 21:45:40 +0200
Arnd Bergmann <arnd@...db.de> wrote:

> On Monday 22 September 2014 21:37:55 Boris BREZILLON wrote:
> > dma_mask and dma_parms are already inherited from the parent device but
> > dma_coherent_mask was left uninitialized (set to zero thanks to kzalloc).
> > Set sub-device coherent_dma_mask to its parent value to simplify
> > sub-drivers making use of dma coherent helper functions (those drivers
> > currently have to explicitly set the dma coherent mask using
> > dma_set_coherent_mask function).
> > 
> > Signed-off-by: Boris BREZILLON <boris.brezillon@...e-electrons.com>
> > ---
> > 
> > Hi,
> > 
> > This patch is follow-up of a discussion we had on a KMS driver thread [1].
> > This patch is only copying the parent device coherent_dma_mask to avoid
> > calling specific dma_set_coherent_mask in case the coherent mask is the
> > default one.
> > 
> > I'm a bit surprised this hasn't been done earlier while other dma fields
> > (mask and parms) are already inherited from the parent device, so please
> > tell me if there already was an attempt to do the same, and if so, what
> > was the reson for rejecting it :-).
> > 
> > 
> 
> Seems reasonable to me. It's not clear whether we should always inherit
> the dma_mask, but I see no point in copying just dma_mask but not
> coherent_dma_mask.

I thought about adding a dma_mask field to mfd_cell to override the
default behavior (allocate a new dma_mask and copy the value 
provided by mfd_cell if it's not zero), but I don't see any real use
case where a sub-device does not share the dma capabilities with its
parent.
IMHO, it's safer to keep it as is until someone really need to set a
different dma_mask on a sub-device.

Best Regards,

Boris



-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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