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Date: Thu, 29 Feb 2024 11:45:07 +0100
From: Daniel Vetter <daniel@ffwll.ch>
To: Jessica Zhang <quic_jesszhan@quicinc.com>
Cc: Maxime Ripard <mripard@kernel.org>,
	Abhinav Kumar <quic_abhinavk@quicinc.com>,
	Jani Nikula <jani.nikula@linux.intel.com>,
	Neil Armstrong <neil.armstrong@linaro.org>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	Sam Ravnborg <sam@ravnborg.org>, dri-devel@lists.freedesktop.org,
	linux-kernel@vger.kernel.org, David Airlie <airlied@gmail.com>,
	Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
	Rob Clark <robdclark@gmail.com>, Rob Herring <robh+dt@kernel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" <devicetree@vger.kernel.org>
Subject: Re: [PATCH RFC 0/4] Support for Simulated Panels
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	Maxime Ripard <mripard@kernel.org>,
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	Jani Nikula <jani.nikula@linux.intel.com>,
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In-Reply-To: <6470960a-dbe8-43ca-889a-b5a7cc934569@quicinc.com>

On Wed, Feb 28, 2024 at 01:49:34PM -0800, Jessica Zhang wrote:
> 
> 
> On 2/2/2024 2:15 AM, Maxime Ripard wrote:
> > On Tue, Jan 30, 2024 at 09:53:13AM +0100, Daniel Vetter wrote:
> > > > > > > > Wouldn't it be simpler if we had a vkms-like panel that we could either
> > > > > > > > configure from DT or from debugfs that would just be registered the
> > > > > > > > usual way and would be the only panel we register?
> > > > > > > 
> > > > > > 
> > > > > > No, we need to have validate actual hardware pipeline with the simulated
> > > > > > panel. With vkms, actual display pipeline will not be validated. With
> > > > > > incorrect display pipeline misconfigurations arising from different panel
> > > > > > combinations, this can easily be caught with any existing IGT CRC testing.
> > > > > > In addition, all performance related bugs can also be easily caught by
> > > > > > simulating high resolution displays.
> > > > > 
> > > > > That's not what I meant. What I meant was that something like a
> > > > > user-configurable, generic, panel driver would be a good idea. Just like
> > > > > vkms (with the debugfs patches) is for a full blown KMS device.
> > > > > 
> > > > 
> > > > Let me respond for both this question and the one below from you/Jani.
> > > > 
> > > > Certainly having user-configurable information is a goal here. The end-goal
> > > > is to make everything there in the existing panels such as below like I
> > > > wrote:
> > > > 
> > > > 1) Display resolution with timings (drm_display_mode)
> > > > 2) Compression/non-compression
> > > > 3) Command mode/Video mode
> > > > 4) MIPI mode flags
> > > > 5) DCS commands for panel enable/disable and other panel sequences
> > > > 6) Power-up/Power-down sequence for the panel
> > > > 
> > > > But, we also have to see what all is feasible today from the DRM fwk
> > > > standpoint. There are some limitations about what is boot-time configurable
> > > > using bootparams and what is runtime configurable (across a modeset) using
> > > > debugfs.
> > > > 
> > > > 1) Today, everything part of struct mipi_dsi_device needs to be available at
> > > > boot time from what I can see as we need that while calling
> > > > mipi_dsi_attach(). So for that we went with boot-params.
> > > > 
> > > > 2) For the list of modes, we can move this to a debugfs like
> > > > "populate_modes" which the client using a sim panel can call before picking
> > > > a mode and triggering a commit.
> > > > 
> > > > But we need to have some default mode and configuration.
> > > 
> > > Uh, at the risk of sounding a bit like I'm just chasing the latest
> > > buzzwords, but this sounds like something that's screaming for ebpf.
> > 
> > I make a half-joke to Jani on IRC about it, but I was also being
> > half-serious. If the goal we want to have is to fully emulate any panel
> > variation, ebpf really looks like the best and most flexible way
> > forward.
> 
> Hi Maxime and Daniel,
> 
> For our current sim panel requirements, we can go with implementing the
> configfs first then add ebpf if requirements get more complex.

Agreed, this is definitely the pragmatic approach to get this going.
-Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch