Re: [RFC 0/2] drm/dsi: DSI for devices with different control bus

From: Lucas Stach <l.stach@pengutronix.de>
To: Thierry Reding <treding@nvidia.com>
Cc: Archit Taneja <architt@codeaurora.org>,
	linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org,
	dri-devel@lists.freedesktop.org, a.hajda@samsung.com
Subject: Re: [RFC 0/2] drm/dsi: DSI for devices with different control bus
Date: Wed, 19 Aug 2015 16:17:08 +0200	[thread overview]
Message-ID: <1439993828.31432.28.camel@pengutronix.de> (raw)
In-Reply-To: <20150819131300.GC15607@ulmo.nvidia.com>

Hi Thierry, Archit,

Am Mittwoch, den 19.08.2015, 15:13 +0200 schrieb Thierry Reding:
> On Wed, Aug 19, 2015 at 10:37:54AM +0530, Archit Taneja wrote:
> > Hi,
> > 
> > On 06/30/2015 10:54 AM, Archit Taneja wrote:
> > >We are currently restricted when it comes to supporting DSI on devices
> > >that have a non-DSI control bus. For example, DSI encoder chips are
> > >available in the market that are configured via i2c. Configuring their
> > >registers via DSI bus is either optional or not available at all.
> > >
> > >These devices still need to pass DSI parameters (data lanes, mode flags
> > >etc) to the DSI host they are connected to. We don't have a way to do
> > >that at the moment.
> > >
> > >The method presented in these patches is to provide an API to create a
> > >'dummy' mipi_dsi_device. This device is populated with the desired DSI
> > >params, which are passed on to the host via mipi_dsi_attach().
> > >
> > >This method will require the device driver to get a phandle to the DSI
> > >host since there is no parent-child relation between the two.
> > >
> > >Is there a better way to do this? Please let me know!
> > 
> > Any comments on this?
> 
> Perhaps a better way would be to invert this relationship. According to
> your proposal we'd have to have DT like this:
> 
> 	i2c@... {
> 		...
> 
> 		dsi-device@... {
> 			...
> 			dsi-bus = <&dsi>;
> 			...
> 		};
> 
> 		...
> 	};
> 
> 	dsi@... {
> 		...
> 	};
> 
> Inversing the relationship would become something like this:
> 
> 	i2c@... {
> 		...
> 	};
> 
> 	dsi@... {
> 		...
> 
> 		peripheral@... {
> 			...
> 			i2c-bus = <&i2c>;
> 			...
> 		};
> 
> 		...
> 	};
> 
> Both of those aren't fundamentally different, and they both have the
> disavantage of lacking ways to transport configuration data that the
> other bus needs to instantiate the dummy device (such as the reg
> property for example, denoting the I2C slave address or the DSI VC).
> 
> So how about we create two devices in the device tree and fuse them at
> the driver level:
> 
> 	i2c@... {
> 		...
> 
> 		i2cdsi: dsi-device@... {
> 			...
> 		};
> 
> 		...
> 	};
> 
> 	dsi@... {
> 		...
> 
> 		peripheral@... {
> 			...
> 			control = <&i2cdsi>;
> 			...
> 		};
> 
> 		...
> 	};
> 
> This way we'll get both an I2C device and a DSI device that we can fully
> describe using the standard device tree bindings. At driver time we can
> get the I2C device from the phandle in the control property of the DSI
> device and use it to execute I2C transactions.
> 
I don't really like to see that you are inventing yet-another-way to
handle devices connected to multiple buses.

Devicetree is structured along the control buses, even if the devices
are connected to multiple buses, in the DT they are always children of
the bus that is used to control their registers from the CPUs
perspective. So a DSI encoder that is controlled through i2c is clearly
a child of the i2c master controller and only of that one.

If you need to model connections between devices that are not reflected
through the control bus hierarchy you should really consider using the
standardized of-graph bindings.
(Documentation/devicetree/bindings/graph.txt)

Multiple device drivers in both the media and DRM universe have shown
that they are a working way to represent those data bus connections
between devices.
I know this might make things a bit more complicated for Tegra DRM,
where you have a nice parent<->child relationship between the components
even on the control path so far, but we should really move into the
direction of more drivers using the standardized bindings for this
stuff, instead of doing another round of NIH.

Regards,
Lucas
-- 
Pengutronix e.K.             | Lucas Stach                 |
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