[PATCH 1/3] staging: imx-drm: document internal HDMI I2C master controller DT binding

Hi Vladimir:

  I am working on convert imx-hdmi to dw_hdmi now:
  https://lkml.org/lkml/2014/12/1/190
  I also have a plan to use  the internal HDMI I2C master under the I2c 
framework,
and I also have a patch to do this work. So glad to see your work.
  Please also Cc me[off-list ref] and Zubair.Kakakhel at imgtec.com,
maybe Zubair also have interests on your future patch.
On 2014?12?02? 00:22, Philipp Zabel wrote:

Hi Vladimir,

[Added Andy Yan to Cc:, because imx-hdmi->dw-hdmi]

Am Montag, den 01.12.2014, 17:39 +0200 schrieb Vladimir Zapolskiy:

quoted

On 01.12.2014 17:11, Philipp Zabel wrote:

quoted

Am Montag, den 01.12.2014, 16:54 +0200 schrieb Vladimir Zapolskiy:

quoted

Hi Philipp and Shawn,

On 15.11.2014 19:49, Vladimir Zapolskiy wrote:

quoted

Provide information about how to bind internal iMX6Q/DL HDMI DDC I2C
master controller. The property is set as optional one, because iMX6
HDMI DDC bus may be represented by one of general purpose I2C busses
found on SoC.

Signed-off-by: Vladimir Zapolskiy <redacted>
Cc: Wolfram Sang <redacted>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Shawn Guo <redacted>
Cc: devicetree at vger.kernel.org
Cc: linux-media at vger.kernel.org
Cc: linux-arm-kernel at lists.infradead.org
Cc: linux-i2c at vger.kernel.org
---
  Documentation/devicetree/bindings/staging/imx-drm/hdmi.txt |   10 +++++++++-
  1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/Documentation/devicetree/bindings/staging/imx-drm/hdmi.txt b/Documentation/devicetree/bindings/staging/imx-drm/hdmi.txt
index 1b756cf..43c8924 100644
--- a/Documentation/devicetree/bindings/staging/imx-drm/hdmi.txt
+++ b/Documentation/devicetree/bindings/staging/imx-drm/hdmi.txt

@@ -10,6 +10,8 @@ Required properties:
   - #address-cells : should be <1>
   - #size-cells : should be <0>
   - compatible : should be "fsl,imx6q-hdmi" or "fsl,imx6dl-hdmi".
+   If internal HDMI DDC I2C master controller is supposed to be used,
+   then "simple-bus" should be added to compatible value.
   - gpr : should be <&gpr>.
     The phandle points to the iomuxc-gpr region containing the HDMI
     multiplexer control register.

@@ -22,6 +24,7 @@ Required properties:
  
  Optional properties:
   - ddc-i2c-bus: phandle of an I2C controller used for DDC EDID probing
+ - ddc: internal HDMI DDC I2C master controller
  
  example:
  

@@ -32,7 +35,7 @@ example:
          hdmi: hdmi at 0120000 {
                  #address-cells = <1>;
                  #size-cells = <0>;
-                compatible = "fsl,imx6q-hdmi";
+                compatible = "fsl,imx6q-hdmi", "simple-bus";
                  reg = <0x00120000 0x9000>;
                  interrupts = <0 115 0x04>;
                  gpr = <&gpr>;

@@ -40,6 +43,11 @@ example:
                  clock-names = "iahb", "isfr";
                  ddc-i2c-bus = <&i2c2>;
  
+                hdmi_ddc: ddc {
+                        compatible = "fsl,imx6q-hdmi-ddc";
+                        status = "disabled";
+                };
+
                  port at 0 {
                          reg = <0>;
  

knowing in advance that I2C framework lacks a graceful support of non
fully compliant I2C devices, do you have any objections to the proposed
iMX HDMI DTS change?

I'm not sure about this. Have you seen "drm: Decouple EDID parsing from
I2C adapter"? I feel like in the absence of a ddc-i2c-bus property the
imx-hdmi/dw-hdmi driver should try to use the internal HDMI DDC I2C
master controller, bypassing the I2C framework altogether.

My idea is exactly not to bypass the I2C framework, briefly the
rationale is that
* it allows to reuse I2C UAPI/tools naturally applied to the internal
iMX HDMI DDC bus,
* it allows to use iMX HDMI DDC bus as an additional feature-limited I2C
bus on SoC (who knows, I absolutely won't be surprised, if anyone needs
it on practice),
* if an HDMI controller supports an external I2C bus, the integration
with HDMI DDC bus driver based on I2C framework is seamless.

However I agree that the selected approach may look odd, the question is
if the oddness comes from the technical side or from the fact that
nobody has done it before this way.

I'm open to any critique, if the proposal of creating an I2C bus from
HDMI DDC bus is lame, then I suppose the shared iMX HDMI DDC bus driver
should be converted to something formless and internally used by
imx-hdmi. The negative side-effects of such a change from my point of
view are
* more or less natural modularity is lost,
* a number of I2C framework API/functions should be copy-pasted to the
updated HDMI DDC bus driver to support a subset of I2C read/write
transactions.

If Wolfram is happy to accomodate such feature limited, 'I2C master'
devices in i2c/drivers/busses in principle, I won't disagree.

But then it should be abstracted properly. The dw-hdmi-tx core on i.MX6
has the DDC I2C master register space at 0x7e00 - 0x7e12. What are the
offsets on the Rockchip version? If the "simple-bus" compatible is to be
set on the hdmi driver, the ddc driver should do its own register
access, and therefore needs a reg property. I suspect for the ddc-i2cm
we should get away with a common compatible like "snps,dw-hdmi-i2c".

	hdmi: hdmi at 120000 {
		/* ... */
		compatible = "fsl,imx6q-hdmi", "snps,dw-hdmi";
		ddc-i2c-bus = <&hdmi_ddc>;

		hdmi_ddc: i2c at 127e00 {
			compatible = "snps,dw-hdmi-i2c";
			reg = <0x1207e00 0x13>
		};

		/* could add phy-i2cm, cec, ... here */
	};

Also there's an i2c bus for communication with the phy at 0x3020 -
0x3032, should that be handled in a similar way, then?

    Rockchip RK3288 has the same DDC I2C master and phy i2c master
    register offset as imx hdmi.

     what my suggestion is: make the ddc-i2c-bus property optional, if 
the dw_hdmi driver
    can't found the ddc-i2c-bus node, then use the internal i2cm:
        ddc_node = of_parse_phandle(np, "ddc-i2c-bus", 0);
         if (ddc_node) {
                 hdmi->ddc = of_find_i2c_adapter_by_node(ddc_node);
                 of_node_put(ddc_node);
                 if (!hdmi->ddc) {
                         dev_dbg(hdmi->dev, "failed to read ddc node\n");
                         return -EPROBE_DEFER;
                 }

         } else {
                 hdmi->ddc = dw_hdmi_ddc_adapter_register(hdmi).
         }

         and we can have a file put with dw_hdmi.c in a same dir, which
        implement  standard i2c adapter interface .
       I found that other hdmi platform such as msm(/msm/hdmi/hdmi_i2c.c) ,
       radeon(radeon_i2c.c) also do like this

Do we need to make the hdmi driver a proper interrupt controller? At
least the two i2c masters have two interrupts each, "done" and "error".

regards
Philipp