Re: [PATCH 4/8] of: dma: Split of_configure_dma() into mask and ops configuration

[Laurent Pinchart <laurent.pinchart-ryLnwIuWjnjg/C1BVhZhaw@public.gmane.org>]

Hi Tomasz,

On Friday 12 Aug 2016 16:31:47 Tomasz Figa wrote:
> On Tue, Aug 9, 2016 at 7:49 AM, Sricharan R wrote:
> > From: Laurent Pinchart <laurent.pinchart+renesas-ryLnwIuWjnjg/C1BVhZhaw@public.gmane.org>
> > 
> > The of_configure_dma() function configures both the DMA masks and ops.
> > Moving DMA ops configuration to probe time would thus also delay
> > configuration of the DMA masks, which might not be safe. To avoid issues
> 
> Do we know any example cases when it might be unsafe? I think we kind
> of rely on the fact that DMA mapping (and so DMA masks as well) is not
> used before probing the device anyway, because we let the IOMMU
> attachment happen at probe time, which essentially makes any earlier
> attempts to use DMA mapping on such device incorrect.

I don't know of any such situation, but (if I remember correctly) when I 
discussed the IOMMU rework with Arnd Bergman and Will Deacon there was a 
concern that someone, somewhere was relying on the mask being set early.

I personally would like to drop this patch, but it might be difficult to 
ensure this wouldn't cause a regression. There should certainly be no DMA 
mapping created before IOMMU attachment, but the DMA mask could possibly be 
used somewhere else.

-- 
Regards,

Laurent Pinchart