[RFC] Using DC in amdgpu for upcoming GPU

[Dave Airlie]

> We would love to upstream DC for all supported asic!  We made enough change
> to make Sea Island work but it's really not validate to the extend we
> validate Polaris on linux and no where close to what we do for 2017 ASICs.
> With DC the display hardware programming, resource optimization, power
> management and interaction with rest of system will be fully validated
> across multiple OSs.  Therefore we have high confidence that the quality is
> going to better than what we have upstreammed today.
>
> I don't have a baseline to say if DC is in good enough quality for older
> generation compare to upstream.  For example we don't have HW generate
> bandwidth_calc for DCE 8/10 (Sea/Vocanic island family) but our code is
> structured in a way that we assume bandwidth_calc is there.  None of us feel
> like go untangle the formulas in windows driver at this point to create our
> own version of bandwidth_calc.  It sort of work with HW default values but
> some mode / config is likely to underflows.  If community is okay with
> uncertain quality, sure we would love to upstream everything to reduce our
> maintaince overhead.  You do get audio with DC on DCE8 though.

If we get any of this upstream, we should get all of the hw supported with it.

If it regresses we just need someone to debug why.

> Maybe let me share what we are doing and see if we can come up with
> something to make DC work for both upstream and our internal need.  We are
> sharing code not just on Linux and we will do our best to make our code
> upstream friendly.  Last year we focussed on having enough code to prove
> that our DAL rewrite works and get more people contributing to it.  We rush
> a bit as a result we had a few legacy component we port from Windows driver
> and we know it's bloat that needed to go.
>
> We designed DC so HW can contribute bandwidth_calc magic and psuedo code to
> program the HW blocks.  The HW blocks on the bottom of DC.JPG in models our
> HW blocks and the programming sequence are provided by HW engineers.  If a
> piece of HW need a bit toggled 7 times during power up I rather have HW
> engineer put that in their psedo code rather than me trying to find that
> sequence in some document.  Afterall they did simulate the HW with the
> toggle sequence.  I guess these are back-end code Daniel talked about.  Can
> we agree that DRM core is not interested in how things are done in that
> layer and we can upstream these as it?
>
> The next is dce_hwseq.c to program the HW blocks in correct sequence.  Some
> HW block can be programmed in any sequence, but some requires strict
> sequence to be followed.  For example Display CLK and PHY CLK need to be up
> before we enable timing generator.  I would like these sequence to remain in
> DC as it's really not DRM's business to know how to program the HW.  In a
> way you can consider hwseq as a helper to commit state to HW.
>
> Above hwseq is the dce*_resource.c.  It's job is to come up with the HW
> state required to realize given config.  For example we would use the exact
> same HW resources with same optimization setting to drive any same given
> config.  If 4 x 4k at 60 is supported with resource setting A on HW diagnositc
> suite during bring up setting B on Linux then we have a problem.  It know
> which HW block work with which block and their capability and limitations.
> I hope you are not asking this stuff to move up to core because in reality
> we should probably hide this in some FW, as HW expose the register to config
> them differently that doesn't mean all combination of HW usage is validated.
> To me resource is more of a helper to put together functional pipeline and
> does not make any decision that any OS might be interested in.
>
> These yellow boxes in DC.JPG are really specific to each generation of HW
> and changes frequently.  These are things that HW has consider hiding it in
> FW before.  Can we agree on those code (under /dc/dce*) can stay?

I think most of these things are fine to be part of the solution we end up at,
but I can't say for certain they won't require interface changes. I think the
most useful code is probably the stuff in the dce subdirectories.

>
> Is this about demonstration how basic functionality work and add more
> features with series of patches to make review eaiser?  If so I don't think
> we are staff to do this kind of rewrite.  For example it make no sense to
> hooking up bandwidth_calc to calculate HW magic if we don't have mem_input
> to program the memory settings.  We need portion of hw_seq to ensure these
> blocks are programming in correct sequence.  We will need to feed
> bandwidth_calc it's required inputs, which is basically the whole system
> state tracked in validate_context today, which means we basically need big
> bulk of resource.c.  This effort might have benefit in reviewing the code,
> but we will end up with pretty much similar if not the same as what we
> already have.

This is something people always say, I'm betting you won't end up there at all,
it's not just review, it's incremental development model, so that when things
go wrong we can pinpoint why and where a lot easier. Just merging this all in
one fell swoop is going to just mean a lot of pain in the end. I understand you
aren't resourced for this sort of development on this codebase, but it's going
to be an impasse to try and merge this all at once even if was clean code.

> Or is the objection that we have the white boxes in DC.JPG instead of using
> DRM objects?  We can probably workout something to have the white boxes
> derive from DRM objects and extend atomic state with our validate_context
> where dce*_resource.c stores the constructed pipelines.

I think Daniel explained quite well how things should look in terms of
subclassing.

>
> 5) Why is a midlayer bad?
> I'm not going to go into specifics on the DC midlayer, but we abhor
> midlayers for a fair few reasons. The main reason I find causes the
> most issues is locking. When you have breaks in code flow between
> multiple layers, but having layers calling back into previous layers
> it becomes near impossible to track who owns the locking and what the
> current locking state is.
>
> Consider
>     drma -> dca -> dcb -> drmb
>     drmc -> dcc  -> dcb -> drmb
>
> We have two codes paths that go back into drmb, now maybe drma has a
> lock taken, but drmc doesn't, but we've no indication when we hit drmb
> of what the context pre entering the DC layer is. This causes all
> kinds of problems. The main requirement is the driver maintains the
> execution flow as much as possible. The only callback behaviour should
> be from an irq or workqueue type situations where you've handed
> execution flow to the hardware to do something and it is getting back
> to you. The pattern we use to get our of this sort of hole is helper
> libraries, we structure code as much as possible as leaf nodes that
> don't call back into the parents if we can avoid it (we don't always
> succeed).
>
> Okay.  by the way DC does behave like a helper for most part.  There is no
> locking in DC.  We work enough with different OS to know they all have
> different synchronization primatives and interrupt handling and have DC lock
> anything is just shooting ourself in the foot.  We do have function with
> lock in their function name in DC but those are HW register lock to ensure
> that the HW register update atomically. ie have 50 register write latch in
> HW at next vsync to ensure HW state change on vsync boundary.
>
> So the above might becomes
>    drma-> dca_helper
>            -> dcb_helper
>            -> drmb.
>
> In this case the code flow is controlled by drma, dca/dcb might be
> modifying data or setting hw state but when we get to drmb it's easy
> to see what data is needs and what locking.
>
> DAL/DC goes against this in so many ways, and when I look at the code
> I'm never sure where to even start pulling the thread to unravel it.
>
> I don't know where we go against it.  In the case we do callback to DRM for
> MST case we have
>
> amdgpu_dm_atomic_commit (implement atomic_commit)
> dc_commit_targets (commit helper)
> dce110_apply_ctx_to_hw (hw_seq)
> core_link_enable_stream (part of MST enable sequence)
> allocate_mst_payload (helper for above func in same file)
> dm_helpers_dp_mst_write_payload_allocation_table (glue code to call DRM)
> drm_dp_mst_allocate_vcpi (DRM)
>
> As you see even in this case we are only 6 level deep before we callback to
> DRM, and 2 of those functions are in same file as helper func of the bigger
> sequence.
>
> Can you clarify the distinction between what you would call a mid layer vs
> helper.  We consulted Alex a lot and we know about this inversion of control
> pattern and we are trying our best to do it.  Is it the way functions are
> named and files folder structure?  Would it help if we flatten
> amdgpu_dm_atomic_commit and dc_commit_targets?  Even if we do I would
> imagine we want some helper in commit rather a giant 1000 line function.  Is
> there any concern that we put dc_commit_targets under /dc folder as we want
> other platform to run exact same helper?  Or this is about the state
> dc_commit_targets is too big?  or the state is stored validate_context
> rather than drm_atomic_state?

Well one area I hit today while looking, is trace the path for a dpcd
read or write.

An internal one in the dc layer goes

core_link_dpcd_read

>
> I don't think it make sense for DRM to get into how we decide to use our HW
> blocks.  For example any refactor done in core should not result in us using
> different pipeline to drive the same config.  We would like to have control
> over how our HW pipeline is constructed.
>
> Some questions I have for AMD engineers that also I'd want to see
> addressed before any consideration of merging would happen!
>
> How do you plan on dealing with people rewriting or removing code
> upstream that is redundant in the kernel, but required for internal
> stuff?
>
>
> Honestly I don't know what these are.  Like you and Jerome remove func ptr
> abstraction (I know it was bad, that was one of the component we ported from
> windows) and we need to keep it as function pointer so we can still run our
> code on FPGA before we see first silicon?  I don't think if we nak the
> function ptr removal will be a problem for community.  The rest is valued
> and we took with open arm.
>
> Or this is more like we have code duplication after DRM added some
> functionality we can use?  I would imaging its more of moving what we got
> working in our code to DRM core if we are upstreamed and we have no problem
> accomodate for that as the code moved out to DRM core can be included in
> other platforms.  We don't have any private ioctl today and we don't plan to
> have any outside of using DRM object properties.
>
>
> How are you going to deal with new Linux things that overlap
> incompatibly with your internally developed stuff?
>
> I really don't know what those new linux things can be that could cause us
> problem.  If anything the new things will be probably come from us if we are
> upstreammed.
>
> atomic: we had that on windows 8 years ago for windows vista, yes
> sematic/abstraction is different but concept is the same. We could have
> easily settled with DRM semantics or DRM could easily take some form of our
> pattern.
>
> DP MST:  AMD was the first source certified and we work closely with the
> first branch certified. I was a part of that team and we had a very solid
> implementation.  If we were upstreamed I don't see you would want to
> reinvent the wheel and not try to massage what we have into shape for DRM
> core for other driver to reuse.
>
> drm_plane: windows multi-plane overlay and Andriod HW composer? We had that
> working 2 years ago.  If you are upstreammed and you are first you usually
> have a say in how it should go down don't you?
>
> The new thing coming are Free Sync HDR, 8k at 60 with DP DSC etc.  I would
> imaging we would beat all other vendor to the first open source solution if
> we leverage effort from our extended display team.
>
> If the code is upstream will it be tested in the kernel by some QA
> group, or will there be some CI infrastructure used to maintain and to
> watch for Linux code that breaks assumptions in the DC code?
>
> We have tester that runs set of display test every other day on linux.  We
> don't run on DRM_Next tree yet and Alex is working out a plan to allow us
> use DRM_Next as our development branch.  Upstream is not likely to be tested
> by QA though.
>
> DC does not assume anything.  DC require full state given in
> dc_commit_targets / dc_commit_surfaces_to_target.  we do whatever is
> specified in the data structure.  dc_commit_surfaces_to_target can be
> considered as a helper function to change plane without visual side effect
> on vsync bondary.  dc_commit_targets can be considered as a helper function
> to light up a display with black screen.  DRM core has full control if you
> want to light up to black screen as soon as monitor is plugged in or you
> want to light up after someone does a mode set.  Hotplug interrupt goes to
> amdgpu_dm, and it will take the require lock in DRM object because calling
> DC to detect.
>
> Can you show me you understand that upstream code is no longer 100% in
> your control and things can happen to it that you might not expect and
> you need to deal with it?
>
> I think so, other than we haven't been spanning the mailing list.  We
> already dealing with we don't control 100% our code to some extend.  We
> don't control bandwidth_calc.  Trust me we are not keeping up with the
> updates that HW is doing with it for next gen hw.  Everytime we pull there
> is a new term they added and we have to find a way to feed that input.  We
> had to clean up linux style for them everytime we pull.  Our HW diagnostic
> suite has different set of requirements and they frequently contribute to
> our code.  We took you and Jerome's patch.  If it's validated we want that
> code.
>
> At end of the day I think the architecture is really about what's HW and
> what's DRM core.  Like I said all the yellow boxes has been proposed to
> running on firmware but we decide to keep them in driver as it's easier to
> debug on x86 than uC.  I can tell you that our HW guys were happy when I
> decide to open source bandwidth_calc but we did it anyways.  I feel like
> because we are opening up the complexity and inner working of our HW, we are
> somehow getting penalized for being open.
>
> Dave.
>
> Tony