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From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     jacopo mondi <jacopo@jmondi.org>
Cc:     Peter Rosin <peda@axentia.se>, linux-kernel@vger.kernel.org,
        David Airlie <airlied@linux.ie>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Nicolas Ferre <nicolas.ferre@microchip.com>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Boris Brezillon <boris.brezillon@free-electrons.com>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Gustavo Padovan <gustavo@padovan.org>,
        Sean Paul <seanpaul@chromium.org>,
        Russell King <linux@armlinux.org.uk>,
        Jacopo Mondi <jacopo+renesas@jmondi.org>,
        dri-devel@lists.freedesktop.org, devicetree@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 0/7] Add tda998x (HDMI) support to atmel-hlcdc
Date:   Fri, 20 Apr 2018 13:18:13 +0300
Message-ID: <6354350.l8DgUyNhLT@avalon>
Organization: Ideas on Board Oy
In-Reply-To: <20180420085235.GI4235@w540>
References: <20180419162751.25223-1-peda@axentia.se> <20180420085235.GI4235@w540>
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Hello,

On Friday, 20 April 2018 11:52:35 EEST jacopo mondi wrote:
> Hi Peter,
>     I've been a bit a pain in the arse for you recently, but please
> bear with me a bit more, and sorry for jumping late on the band wagon.
> 
> On Thu, Apr 19, 2018 at 06:27:44PM +0200, Peter Rosin wrote:
> > Hi!
> > 
> > I naively thought that since there was support for both nxp,tda19988 (in
> > the tda998x driver) and the atmel-hlcdc, things would be a smooth ride.
> > But it wasn't, so I started looking around and realized I had to fix
> > things.
> > 
> > In v1 and v2 I fixed things by making the atmel-hlcdc driver a master
> > component, but now in v3 I fix things by making the tda998x driver
> > a bridge instead. This was after a suggestion from Boris Brezillion
> > (the atmel-hlcdc maintainer), so there was some risk of bias ... but
> > after comparing what was needed, I too find the bridge approach better.
> > 
> > In addition to the above, our PCB interface between the SAMA5D3 and the
> > HDMI encoder is only using 16 bits, and this has to be described
> > somewhere, or the atmel-hlcdc driver have no chance of selecting the
> > correct output mode. Since I have similar problems with a ds90c185 lvds
> > encoder I added patches to override the atmel-hlcdc output format via
> > DT properties compatible with the media video-interface binding and
> > things start to play together.
> > 
> > Since this series superseeds the bridge series [1], I have included the
> > leftover bindings patch for the ti,ds90c185 here.
> 
> I feel like this series would look better if it would make use of the
> proposed bridge media bus format support I have recently sent out [1]
> (and which was not there when you first sent v1).
> 
> I understand your fundamental problem here is that the bus format
> that should be reported by your bridge is different from the ones
> actually supported by the TDA19988 chip, as the wirings ground some
> of the input pins.
> 
> Although this is defintely something that could be described in the
> bridge's own OF node with the 'bus_width' property, and what I would do,
> now that you have made a bridge out from the tda19988 driver, is:
> 
> 1) Set the bridge accepted input bus_format parsing its pin
> configuration, or default it if that's not implemented yet.
> This will likely be rgb888. You can do that using the trivial
> support for bridge input image formats implemented by my series.
> 2) Specify in the bridge endpoint a 'bus_width' of <16>
> 3) In your atmel-hlcd get both the image format of the bridge (rgb888)
> and parse the remote endpoint property 'bus_width' and get the <16>
> value back.

Parsing properties of remote nodes should be avoided as much as possible, as 
they need to be interpreted in the context of the DT bindings related to the 
compatible string applicable to that node. I'd rather have the bus_width 
property in the local endpoint node.

> 4) Set the correct format in the atmel hlcd driver to accommodate a
> bridge that wants rgb888 on a 16 bit wide bus (that would be rgb565,
> or are there other possible combinations I am missing?)
> 
> I would consider this better mostly because in this series you are
> creating a semantic for the whole DRM subsystem on the 'bus_width'
> property, where numerical values as '12', '16' etc are arbitrary tied
> to the selection of a media bus format. At least you should use a
> common set of defines [1] between the device tree and the driver,
> but this looks anyway fragile imho.

This I agree with though. Combining the remote bus format with the local bus 
width should fix the problem without having to parse remote properties.

> Have I maybe missed some parts of the problem you are trying to solve
> here?
> 
> Thank you
>    j
> 
> [1] drm: bridge: Add support for static image formats
>     https://lwn.net/Articles/752296/
> [2] include/uapi/linux/media-bus-format.h
> 
> > Anyway, this series solves some real issues for my HW.
> > 
> > Cheers,
> > Peter
> > 
> > Changes since v2   https://lkml.org/lkml/2018/4/17/385
> > - patch 2/7 fixed spelling and added an example
> > - patch 4/7 parse the DT up front and store the result indexed by encoder
> > - old patch 5/6 and 6/6 dropped
> > - patch 5-7/7 are new and makes the tda998x driver a drm_bridge
> > 
> > Changes since v1   https://lkml.org/lkml/2018/4/9/294
> > - added reviewed-by from Rob to patch 1/6
> > - patch 2/6 changed so that the bus format override is in the endpoint
> >   DT node, and follows the binding of media video-interfaces.
> > - patch 3/6 is new, it adds drm_of_media_bus_fmt which parses above
> >   media video-interface binding (partially).
> > - patch 4/6 now makes use of the above helper (and also fixes problems
> >   with the 3/5 patch from v1 when no override was specified).
> > - do not mention unrelated connector display_info details in the cover
> >   letter and commit messages.
> > 
> > [1]
> > "Bridge" series v2   https://lkml.org/lkml/2018/3/26/610
> > "Bridge" series v1   https://lkml.org/lkml/2018/3/17/221
> > 
> > Peter Rosin (7):
> >   dt-bindings: display: bridge: lvds-transmitter: add ti,ds90c185
> >   dt-bindings: display: atmel: optional video-interface of endpoints
> >   drm: of: introduce drm_of_media_bus_fmt
> >   drm/atmel-hlcdc: support bus-width (12/16/18/24) in endpoint nodes
> >   drm/i2c: tda998x: find the drm_device via the drm_connector
> >   drm/i2c: tda998x: split encoder and component functions from the work
> >   drm/i2c: tda998x: register as a drm bridge
> >  
> >  .../devicetree/bindings/display/atmel/hlcdc-dc.txt |  26 +++
> >  .../bindings/display/bridge/lvds-transmitter.txt   |   8 +-
> >  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c     |  71 ++++++--
> >  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_dc.h       |   2 +
> >  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_output.c   |  40 +++-
> >  drivers/gpu/drm/drm_of.c                           |  38 ++++
> >  drivers/gpu/drm/i2c/tda998x_drv.c                  | 201 ++++++++++++++--
> >  include/drm/drm_of.h                               |   7 +
> >  8 files changed, 342 insertions(+), 51 deletions(-)

-- 
Regards,

Laurent Pinchart