[RFC] Driver configuration using Device Tree

[Eric Holmberg]

I am trying to determine if Device Tree is an appropriate use for 
configuring drivers and would like to request comments.  We currently 
use Device Tree in our Shared Memory Driver (SMD) that manages up to 64 
ports (where a port consists of an RX FIFO and a TX FIFO) between any 
two processors and a pair of interrupts for each processor.  The shared 
memory address and interrupt configuration is stored in Device Tree and 
since this is hardware, this is considered an acceptable use.  However, 
we also have two separate modules that use SMD and export devices to 
userspace through either the TTY framework (SMD TTY) or through a 
character device (SMD PKT).  For these drivers, the configuration has 
less to do with hardware and more about which port to connect to in the 
SMD driver and how to expose the port to userspace through a device 
node.  This code is used in Linux-based phones.

The DT configuration looks like this:
         qcom,smdtty {
                 compatible = "qcom,smdtty";

                 qcom,smdtty-ds {
                         qcom,smdtty-port-name = "DS";
                         qcom,smdtty-edge = <0>;
                         qcom,smdtty-dev-idx = <0>;
                 };
                 . . .
                /* on the order of 10 more port config items */
       };


Question
--------
Is there a concern that DT should only be used for hardware 
configuration and that this "driver configuration" is not an acceptable 
use?  If it is not acceptable, should I go back to using platform 
devices (seems like a step backwards) or some other method such as 
exporting a control channel to userspace that can be configured using an 
IOCTL?


More Detailed Background Information
------------------------------------
Here’s a diagram showing the modules to help visualize the system.

                   +-------------------------------+
                   |                               |
                   |    SMD Userspace Clients      |
                   |                               |
                   +-------------------------------+
    User space              ^              ^
  +-------------------------|--------------|----------------------+
    Kernel space            +              +
                      +--------------------------+
                      | Linux Virtual File Sys   |
                      |                          |
                      +--------------------------+
                            ^              ^
                            |              |
                            |        +---------------+
                            |        |    Linux      |
                            |        | TTY Framework |
                            |        +---------------+
                            |              ^
                            |              |
     +---------+            |              |               +---------+
     |   DT    |      +-----------+  +-----------+         |   DT    |
     |  Config |+---->|    SMD    |  |    SMD    |<-------+|  Config |
     |         |      |    PKT    |  |    TTY    |         |         |
     +---------+      +-----------+  +-----------+         +---------+
                            |              |
                            |              |
                            v              |
                            O <------------+
                            |
     +---------+            |
     |   DT    |      +------------------------+
     |  Config |+---->|          SMD           |
     |         |      |                        |
     +---------+      +------------------------+
                         |             ^   |
                         |             |   |
                         |             |   |
                         |             |   |
                         |             |   |
                         |             |   |
                         v             |   |
               +--------------------+  |   |Interrupts
               |                    |  |   |
               |  Shared Memory     |  |   |
               |                    |  |   |
               +---------+----------+  |   |
                         |             |   |
                         |             |   |
                         |             |   |
                         |             |   |
                         |             |   |
                         |             |   |
                         v             |   v
          +-----------------------------------+
          |                                   |
          |     Remote System (Modem, etc)    |
          |                                   |
          +-----------------------------------+

Here’s an example of our DT schema.  Each SMD channel is uniquely 
identified in the system by the NAME:EDGE, so this is included in the 
configuration along with the index or name of the device node to export 
to userspace (either a TTY device for streaming-mode data or a character 
device for packet-mode data).

[Root level node]
Required properties:
	-compatible: should be "qcom,smdtty"

[Second level nodes]
qcom,smdtty-port-names
Required properties:
	-qcom,smdtty-port-name: the name that identifies the smd channel
	-qcom,smdtty-edge: the smd edge (this is an enumeration that
			identifies the local<->remote processor edge)
	-qcom,smdtty-dev-idx: the device node index

Example:

         qcom,smdtty {
                 compatible = "qcom,smdtty";

                 qcom,smdtty-ds {
                         qcom,smdtty-port-name = "DS";
                         qcom,smdtty-edge = <0>;
                         qcom,smdtty-dev-name = <0>;
                 };

                 qcom,smdtty-apps-fm {
                         qcom,smdtty-port-name = "APPS_FM";
                         qcom,smdtty-edge = <6>;
                         qcom,smdtty-dev-name = <1>;
                 };

                 qcom,smdtty-gpsnmea {
                         qcom,smdtty-port-name = "GPSNMEA";
                         qcom,smdtty-edge = <0>;
                         qcom,smdtty-dev-name = <27>;
                 };
         };

Thanks,

Eric Holmberg
-- 
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