Re: [PATCH v4 1/5] drm/rockchip: Add basic drm driver

[Arnd Bergmann]

On Monday 22 September 2014 17:15:06 Boris BREZILLON wrote:
> > > +
> > > +   /* TODO(djkurtz): fetch the mapping start/size from somewhere */
> > > +   mapping = arm_iommu_create_mapping(&platform_bus_type, 0x10000000,
> > > +                                      SZ_1G);
> > > +   if (IS_ERR(mapping)) {
> > > +           ret = PTR_ERR(mapping);
> > > +           goto err_config_cleanup;
> > > +   }
> > > +
> > > +   dma_set_coherent_mask(dev, DMA_BIT_MASK(32));
> > 
> > This is the default coherent mask. If you call this function, you
> > should normally check the return value, or call dma_set_mask first,
> > which you apparently don't do here, and in another place in this
> > patch.
> 
> By "This is the default mask" do you mean it shouldn't be called at
> all ? Cause I ran into some trouble when not calling this in my
> atmel-hlcdc driver.

We used to get this wrong in the of_platform code, but it should
work now.

> Actually, in my case the platform device is created by the MFD core
> which seems to let the coherent_dma_mask uninitialized.

That may well be different, but it seems like a good idea to allow
the MFD core to set this up as well.

In general, we expect that devices that are capable of doing DMA
start with a 32-bit mask for both dma_mask and dma_coherent_mask,
and a driver that requires a smaller masks or wants a larger mask
has to call the appropriate interface to set these, and check the
return value.

	Arnd
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