linux-kernel - Re: RE: Re: [PATCH 0/4] Fixing live video input in ZynqMP DPSUB

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Message-ID: <2ytxhpti53e743b5pca3oa5jmscffi4vpsyeh727bcoh4v6cuw@zkz5pqkcv7v2>
Date: Thu, 1 Feb 2024 18:01:01 +0100
From: Maxime Ripard <mripard@...nel.org>
To: "Klymenko, Anatoliy" <Anatoliy.Klymenko@....com>
Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>, 
	"maarten.lankhorst@...ux.intel.com" <maarten.lankhorst@...ux.intel.com>, "tzimmermann@...e.de" <tzimmermann@...e.de>, 
	"airlied@...il.com" <airlied@...il.com>, "daniel@...ll.ch" <daniel@...ll.ch>, 
	"Simek, Michal" <michal.simek@....com>, 
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: RE: Re: [PATCH 0/4] Fixing live video input in ZynqMP DPSUB

On Fri, Jan 26, 2024 at 11:18:30PM +0000, Klymenko, Anatoliy wrote:
> 
> 
> > -----Original Message-----
> > From: Maxime Ripard <mripard@...nel.org>
> > Sent: Friday, January 26, 2024 4:26 AM
> > To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> > Cc: Klymenko, Anatoliy <Anatoliy.Klymenko@....com>;
> > maarten.lankhorst@...ux.intel.com; tzimmermann@...e.de; airlied@...il.com;
> > daniel@...ll.ch; Simek, Michal <michal.simek@....com>; dri-
> > devel@...ts.freedesktop.org; linux-arm-kernel@...ts.infradead.org; linux-
> > kernel@...r.kernel.org
> > Subject: Re: Re: [PATCH 0/4] Fixing live video input in ZynqMP DPSUB
> > 
> > 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.

Maxime

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