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

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Date:	Mon, 22 Sep 2014 17:54:42 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Boris BREZILLON <boris.brezillon@...e-electrons.com>
Cc:	Mark yao <mark.yao@...k-chips.com>, heiko@...ech.de,
	David Airlie <airlied@...il.com>,
	Rob Clark <robdclark@...il.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Grant Likely <grant.likely@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	John Stultz <john.stultz@...aro.org>,
	Rom Lemarchand <romlem@...gle.com>, devicetree@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linux-api@...r.kernel.org,
	linux-rockchip@...ts.infradead.org, dianders@...omium.org,
	marcheu@...omium.org, dbehr@...omium.org, olof@...om.net,
	djkurtz@...omium.org, xjq@...k-chips.com, kfx@...k-chips.com,
	cym@...k-chips.com, cf@...k-chips.com, zyw@...k-chips.com,
	xxm@...k-chips.com, huangtao@...k-chips.com,
	kever.yang@...k-chips.com, yxj@...k-chips.com, wxt@...k-chips.com,
	xw@...k-chips.com
Subject: Re: [PATCH v4 1/5] drm/rockchip: Add basic drm driver

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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