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Date: Sun, 4 Feb 2024 11:56:18 +0200
From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Maxime Ripard <mripard@kernel.org>
Cc: "Klymenko, Anatoliy" <Anatoliy.Klymenko@amd.com>,
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Subject: Re: RE: Re: [PATCH 0/4] Fixing live video input in ZynqMP DPSUB
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On Thu, Feb 01, 2024 at 06:01:01PM +0100, Maxime Ripard wrote:
> On Fri, Jan 26, 2024 at 11:18:30PM +0000, Klymenko, Anatoliy wrote:
> > On Friday, January 26, 2024 4:26 AM, Maxime Ripard wrote:
> > > On Wed, Jan 17, 2024 at 04:23:43PM +0200, Laurent Pinchart wrote:
> > > > On Mon, Jan 15, 2024 at 09:28:39AM +0100, Maxime Ripard wrote:
> > > > > On Fri, Jan 12, 2024 at 03:42:18PM -0800, Anatoliy Klymenko wrote:
> > > > > > Patches 1/4,2/4,3/4 are minor fixes.
> > > > > >
> > > > > > DPSUB requires input live video format to be configured.
> > > > > > Patch 4/4: The DP Subsystem requires the input live video format to be
> > > > > > configured. In this patch we are assuming that the CRTC's bus format is fixed
> > > > > > and comes from the device tree. This is a proposed solution, as there are no api
> > > > > > to query CRTC output bus format.
> > > > > >
> > > > > > Is this a good approach to go with?
> > > > >
> > > > > I guess you would need to expand a bit on what "live video input" is? Is
> > > > > it some kind of mechanism to bypass memory and take your pixels straight
> > > > > from a FIFO from another device, or something else?
> > > >
> > > > Yes and no.
> > > >
> > > > The DPSUB integrates DMA engines, a blending engine (two planes), and a
> > > > DP encoder. The dpsub driver supports all of this, and creates a DRM
> > > > device. The DP encoder hardware always takes its input data from the
> > > > output of the blending engine.
> > > >
> > > > The blending engine can optionally take input data from a bus connected
> > > > to the FPGA fabric, instead of taking it from the DPSUB internal DMA
> > > > engines. When operating in that mode, the dpsub driver exposes the DP
> > > > encoder as a bridge, and internally programs the blending engine to
> > > > disable blending. Typically, the FPGA fabric will then contain a CRTC of
> > > > some sort, with a driver that will acquire the DP encoder bridge as
> > > > usually done.
> > > >
> > > > In this mode of operation, it is typical for the IP cores in FPGA fabric
> > > > to be synthesized with a fixed format (as that saves resources), while
> > > > the DPSUB supports multiple input formats.
> > > 
> > > Where is that CRTC driver? It's not clear to me why the format would
> > > need to be in the device tree at all. Format negociation between the
> > > CRTC and whatever comes next is already done in a number of drivers so
> > > it would be useful to have that kind of API outside of the bridge
> > > support.
> >
> > One example of such CRTC driver:
> > https://github.com/Xilinx/linux-xlnx/blob/master/drivers/gpu/drm/xlnx/xlnx_mixer.c It's not
> > upstreamed yet. Bus format negotiations here are handled by utilizing Xilinx-specific bridge
> > framework. Ideally, it would be nice to rework this to comply with the upstream DRM bridge
> > framework.
> >
> > > > Bridge drivers in the upstream kernel work the other way around, with
> > > > the bridge hardware supporting a limited set of formats, and the CRTC
> > > > then being programmed with whatever the bridges chain needs. Here, the
> > > > negotiation needs to go the other way around, as the CRTC is the
> > > > limiting factor, not the bridge.
> > > 
> > > Sounds like there's something to rework in the API then?
> > > 
> > Adding an optional CRTC callback imposing CRTC specific bus format restrictions, which may be
> > called from here https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/drm_bridge.c#L935
> > would solve the problem.
> 
> CRTCs and bridges are orthogonal. If anything, I'd expect that callback
> to be set at the CRTC, encoder and connector levels and filled by the
> drm_bridge code if relevant.

I'm thinking about a new CRTC operation that would be called by the
bridge chain format negotiation helper
drm_atomic_bridge_chain_select_bus_fmts() (or one of the functions it
calls), to filter the list of formats supported by the chain based on
what the CRTC supports, or possibly to pick a format in that list. This
needs to be prototyped

-- 
Regards,

Laurent Pinchart