[PATCH RFC 2/8] DRM: Armada: Add Armada DRM driver

Rob Clark robdclark at gmail.com
Wed Jun 12 16:04:50 EDT 2013

On Wed, Jun 12, 2013 at 1:05 PM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Wed, Jun 12, 2013 at 05:49:14PM +0100, Russell King - ARM Linux wrote:
>> On Wed, Jun 12, 2013 at 09:56:22AM -0400, Rob Clark wrote:
>> > On Wed, Jun 12, 2013 at 9:48 AM, Russell King - ARM Linux
>> > <linux at arm.linux.org.uk> wrote:
>> > > And having thought about this driver, DRM some more, I'm now of the
>> > > opinion that DRM is not suitable for driving hardware where the GPU is
>> > > an entirely separate IP block from the display side.
>> > >
>> > > DRM is modelled after the PC setup where your "graphics card" does
>> > > everything - it has the GPU, display and connectors all integrated
>> > > together.  This is not the case on embedded SoCs, which can be a
>> > > collection of different IPs all integrated together.
>> >
>> > actually it isn't even the case on desktop/laptop anymore, where you
>> > can have one gpu with scanout and a second one without (or just with
>> > display controller not hooked up to anything, etc, etc)
>> >
>> > That is the point of dmabuf and the upcoming fence/reservation stuff.
>> Okay, but dmabuf really needs to be fixed, because as it stands this API
>> is really quite broken wrt the DMA API.  dma_map_sg() is (a) not supposed
>> to have its return value ignored - mappings can fail, and (b) the returned
>> number indicates how many entries are valid for the _mapped_ version of
>> the scatterlist.
>> Both these points are important if your DMA API implementation uses an
>> IOMMU, which may coalesce the scatterlist array when creating mappings -
>> much as described in Documentation/DMA-API.txt and
>> Documentation/DMA-API-HOWTO.txt.
>> That is not all DRMs fault - (a) is attributable to DRM's prime
>> implementation:
>>         sgt = obj->dev->driver->gem_prime_get_sg_table(obj);
>>         if (!IS_ERR_OR_NULL(sgt))
>>                 dma_map_sg(attach->dev, sgt->sgl, sgt->nents, dir);
>> and quite why it does the dma_map_sg() beneath the struct_mutex is
>> beyond me - if the array of pages isn't safe without the mutex being
>> held, then it isn't safe after the dma_map_sg() operation has completed
>> and the mutex has been released.
>> However, (b) is more a problem for dmabuf (which I just rather aptly
>> mistyped as dmabug) because either dmabuf's .map_dma_buf method needs
>> to return the value from dma_map_sg(), or it needs to stop requiring
>> this of the .map_dma_buf method and have it done by the caller of
>> this method so the caller can have access to that returned value.
>> Added Sumit Semwal to this email for the dmabuf issue.
>> Thankfully, this being correct isn't a requirement for me, but it's
>> something which _should_ be fixed.

(just some quick answers.. I'll read rest of thread a bit later when I
have some time)

> Okay, so Sumit Semwal has been a victim of TI's cuts, so don't expect
> dma_buf to get sorted by the original author...

Sumit is at linaro, and should still be caring for dma_buf (although
w/ different email addr)

> Now we come to this:
> drivers/gpu/drm/exynos/exynos_drm_dmabuf.c: return dma_buf_export(exynos_gem_obj, &exynos_dmabuf_ops,
> drivers/gpu/drm/exynos/exynos_drm_dmabuf.c-                         exynos_gem_obj->base.size, flags);
> drivers/gpu/drm/omapdrm/omap_gem_dmabuf.c:  return dma_buf_export(obj, &omap_dmabuf_ops, obj->size, flags);
> drivers/gpu/drm/drm_prime.c:        return dma_buf_export(obj, &drm_gem_prime_dmabuf_ops, obj->size,
> drivers/gpu/drm/drm_prime.c-                              0600);
> drivers/gpu/drm/i915/i915_gem_dmabuf.c:     return dma_buf_export(obj, &i915_dmabuf_ops, obj->base.size, flags);

the drm_prime.c version was a more recent addition of "dmabuf
helpers".. not all the drivers use 'em (which seems to be a good

> Of the three implementations which don't use the generic version, they all
> pass 'flags' to dma_buf_export.  drm_prime.c doesn't, it passes a fixed
> file mode.  What's the correct version, or is flags | 0600 actually the
> right one (as flags only contains O_CLOEXEC)?

the drm_prime version is wrong.. the O_CLOEXEC flag should have been
passed along from what userspace requested when exporting the dmabuf


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