Re: [RFC 09/10] drm/tegra: Add IOMMU support

[Hiroshi Doyu <hdoyu-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>]

Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> writes:

> On Friday 27 June 2014 12:46:14 Hiroshi DOyu wrote:
>> 
>> Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> writes:
>> 
>> > From: Thierry Reding <treding-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
>> >
>> > When an IOMMU device is available on the platform bus, allocate an IOMMU
>> > domain and attach the display controllers to it. The display controllers
>> > can then scan out non-contiguous buffers by mapping them through the
>> > IOMMU.
>> >
>> > Signed-off-by: Thierry Reding <treding-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
>> > ---
>> > @@ -1283,8 +1284,18 @@ static int tegra_dc_init(struct host1x_client *client)
>> >  {
>> >         struct drm_device *drm = dev_get_drvdata(client->parent);
>> >         struct tegra_dc *dc = host1x_client_to_dc(client);
>> > +       struct tegra_drm *tegra = drm->dev_private;
>> >         int err;
>> >
>> > +       if (tegra->domain) {
>> > +               err = iommu_attach_device(tegra->domain, dc->dev);
>> 
>> I wanted to keep device drivers iommu-free with the following:
>> 
>> http://patchwork.ozlabs.org/patch/354074/
>> 
>
> We definitely need something like your series to make iommus work transparently
> on ARM for normal devices, using the of_dma_configure() to look up the correct
> iommu per device and initialize it.

OK

> However, any devices that work with multiple iommu domains cannot do that
> and still need to use the iommu API directy. I believe the tegra drm code
> falls into this category.

I think that the similar thing can be said to "DMA API" vs "IOMMU
API". Most of traditional devices would be ok with the existing DMA API
which can be backed by IOMMU. OTOH, some smart devices may need to dive
into IOMMU API for more precise control of it.