Re: [PATCH 1/7] drm/vc4: Add devicetree bindings for VC4.

[Rob Herring]

On Tue, Aug 25, 2015 at 3:42 PM, Rob Clark <robdclark@xxxxxxxxx> wrote:
> On Mon, Aug 24, 2015 at 9:47 AM, Rob Herring <robherring2@xxxxxxxxx> wrote:
>> On Mon, Aug 17, 2015 at 1:30 PM, Eric Anholt <eric@xxxxxxxxxx> wrote:
>>> Stephen Warren <swarren@xxxxxxxxxxxxx> writes:
>>>
>>>> On 08/12/2015 06:56 PM, Eric Anholt wrote:
>>>>> Signed-off-by: Eric Anholt <eric@xxxxxxxxxx>
>>>>
>>>> This one definitely needs a patch description, since someone might not
>>>> know what a VC4 is, and "git log" won't show the text from the binding
>>>> doc itself. I'd suggest adding the initial paragraph of the binding doc
>>>> as the patch description, or more.
>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt b/Documentation/devicetree/bindings/gpu/brcm,bcm-vc4.txt
>>>
>>>>> +- hvss:             List of references to HVS video scalers
>>>>> +- encoders: List of references to output encoders (HDMI, SDTV)
>>>>
>>>> Would it make sense to make all those nodes child node of the vc4
>>>> object. That way, there's no need to have these lists of objects; they
>>>> can be automatically built up as the DT is enumerated. I know that e.g.
>>>> the NVIDIA Tegra host1x binding works this way, and I think it may have
>>>> been inspired by other similar cases.
>>>
>>> I've looked at tegra, and the component system used by msm appears to be
>>> nicer than it.  To follow tegra's model, it looks like I need to build
>>> this extra bus thing corresponding to host1x that is effectively the
>>> drivers/base/component.c code, so that I can get at vc4's structure from
>>> the component drivers.
>>>
>>>> Of course, this is only appropriate if the HW modules really are
>>>> logically children of the VC4 HW module. Perhaps they aren't. If they
>>>> aren't though, I wonder what this "vc4" module actually represents in HW?
>>>
>>> It's the subsystem, same as we use a subsystem node for msm, sti,
>>> rockchip, imx, and exynos.  This appears to be the common model of how
>>> the collection of graphics-related components is represented in the DT.
>>
>> I think most of these bindings are wrong. They are grouped together
>> because that is what DRM wants not because that reflects the h/w. So
>> convince me this is one block, not that it is what other people do.
>
> I think, when it comes to more complex driver subsystems (like drm in
> particular) we have a bit of mismatch between how things look from the
> "pure hw ignoring sw" perspective, and the "how sw and in particular
> userspace expects things" perspective.  Maybe it is less a problem in
> other subsystems, where bindings map to things that are only visible
> in the kernel, or well defined devices like uart or sata controller.
> But when given the choice, I'm going to err on the side of not
> confusing userspace and the large software stack that sits above
> drm/kms, over dt purity.

I wasn't implying that this should get exposed to userspace as
components. V4L2 has gone that route with media controller and
sub-devs. Perhaps that is needed for DRM, perhaps not. For the moment,
I definitely agree the kernel should hide most/all of those details,
but I don't think that means DT has to hide the details or know what
components are handled by a single driver. My point was that on the DT
side we have a mixture of OF graph usage, parent-child nodes or custom
phandles (this case) to describe the relationships between h/w
components. That's not necessarily wrong, but we should have some
rules around how certain relationships are described. Then in the
drivers we have a mixture of deferred probe, component API, and custom
inter-module APIs to control init order. We then have a mixture of all
those which leads to very few if any drivers having the same overall
structure that could be shared. Should we mandate using the component
API for h/w that is discrete blocks? Should we throw out the component
API for something else? Can we tie the graph parsing and component API
together with common code?


> Maybe it would be nice to have a sort of dt overlay that adds the bits
> needed to tie together hw blocks that should be assembled into a
> single logical device for linux and userspace (but maybe not some
> other hypothetical operating system).  But so far that doesn't exist.

OF graph is supposed to do this. OF graph is a double edged sword. It
is very flexible, but then each platform can do something different.
We need to have some level of requirements around how the OF graph is
used. As an example, any system with an HDMI connector should have an
"hdmi-connector" compatible node or encoder/bridge chips/blocks must
have certain ports defined.


> All we have is a hammer (devicetree), everything looks like a nail.
> End result is we end up adding some things in the bindings which
> aren't purely about the hw.  Until someone invents a screwdriver, I'm
> not sure what else to do.  In the end, other hypothetical OS is free
> to ignore those extra fields in the bindings if it doesn't need them.
> So meh?

We really want to err on the side of fewer bindings, not more as once
used they are an ABI.

Rob
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