Re: [PATCH] mfd: inherit coherent_dma_mask from parent device

[Boris BREZILLON]

Hi Arnd,

On Mon, 22 Sep 2014 21:45:40 +0200
Arnd Bergmann <arnd@xxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> > ---
> > 
> > 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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