Re: [PATCH 3/6] dt/bindings: Add bindings for Tegra GMI controller

Mirza Krak
2016-08-09T08:40:22Z

2016-08-08 16:44 GMT+02:00 Jon Hunter <jonathanh@nvidia.com>:
>
> On 06/08/16 20:40, Mirza Krak wrote:
>> From: Mirza Krak <mirza.krak@gmail.com>
>>
>> Document the devicetree bindings for the Generic Memory Interface (GMI)
>> bus driver found on Tegra SOCs.
>>
>> Signed-off-by: Mirza Krak <mirza.krak@gmail.com>
>> ---
>> .../devicetree/bindings/bus/nvidia,tegra20-gmi.txt | 99 ++++++++++++++++++++++
>> 1 file changed, 99 insertions(+)
>> create mode 100644 Documentation/devicetree/bindings/bus/nvidia,tegra20-gmi.txt
>>
>> diff --git a/Documentation/devicetree/bindings/bus/nvidia,tegra20-gmi.txt b/Documentation/devicetree/bindings/bus/nvidia,tegra20-gmi.txt
>> new file mode 100644
>> index 0000000..046846e
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/bus/nvidia,tegra20-gmi.txt
>> @@ -0,0 +1,99 @@
>> +Device tree bindings for NVIDIA Tegra Generic Memory Interface bus
>> +
>> +The Generic Memory Interface bus enables memory transfers between internal and
>> +external memory. Can be used to attach various high speed devices such as
>> +synchronous/asynchronous NOR, FPGA, UARTS and more.
>> +
>> +The actual devices are instantiated from the child nodes of a GMI node.
>> +
>> +Required properties:
>> + - compatible : Should contain one of the following:
>> + For Tegra20 must contain "nvidia,tegra20-gmi".
>> + For Tegra30 must contain "nvidia,tegra30-gmi".
>> + - reg: Should contain GMI controller registers location and length.
>> + - clocks: Must contain an entry for each entry in clock-names.
>> + - clock-names: Must include the following entries: "gmi"
>> + - resets : Must contain an entry for each entry in reset-names.
>> + - reset-names : Must include the following entries: "gmi"
>> + - #address-cells: The number of cells used to represent physical base
>> + addresses in the GMI address space.
>> + - #size-cells: The number of cells used to represent the size of an address
>> + range in the GMI address space.
>> + - ranges: Mapping of the GMI address space to the CPU address space.
>> +
>> +Note that the GMI controller does not have any internal chip-select address
>> +decoding and if you want to access multiple devices external chip-select
>> +decoding must be provided. Furthermore, if you do have external logic to
>
> The above is not 100% accurate. I would say that because there is no
> chip-select address decoding, chip-selects either need to be managed via
> software or by employing external chip-select decoding logic.

ACK, will update with the possibility of managing CS in software.
>
>> +support multiple devices this would assume that the devices use the same
>> +timing and so are probably the same type. It also assumes that they can fit in
>> +the 256MB address range.
>
> Again this is only true for the case where you have external chip-select
> decoding logic, but would not be the case if software were to manage the
> chip-selects.

ACK

>
>> +
>> +Optional properties:
>> +
>> + - nvidia,snor-data-width-32bit: Use 32bit data-bus, default is 16bit.
>> + - nvidia,snor-mux-mode: Enable address/data MUX mode.
>> + - nvidia,snor-rdy-active-before-data: Assert RDY signal one cycle before data.
>> + If omitted it will be asserted with data.
>> + - nvidia,snor-rdy-inv: RDY signal is active high
>> + - nvidia,snor-adv-inv: ADV signal is active high
>> + - nvidia,snor-oe-inv: WE/OE signal is active high
>> + - nvidia,snor-cs-inv: CS signal is active high
>> + - nvidia,snor-cs-select: CS output pin configuration. Default is CS0
>
> Nit ... I think "nvidia,snor-cs" is sufficient for the name. But I am
> not sure if we even need this. See below.
>
>> + <0> : CS0
>> + <1> : CS1
>> + <2> : CS2
>> + <3> : CS3
>> + <4> : CS4
>> + <5> : CS5
>> + <6> : CS6
>> + <7> : CS7
>> +
>> + Note that there is some special handling for the timing values.
>> + From Tegra TRM:
>> + Programming 0 means 1 clock cycle: actual cycle = programmed cycle + 1
>> +
>> + - nvidia,snor-muxed-width: Number of cycles MUX address/data asserted on the
>> + bus. Valid values are 0-15, default is 1
>> + - nvidia,snor-hold-width: Number of cycles CE stays asserted after the
>> + de-assertion of WR_N (in case of SLAVE/MASTER Request) or OE_N
>> + (in case of MASTER Request). Valid values are 0-15, default is 1
>> + - nvidia,snor-adv-width: Number of cycles during which ADV stays asserted.
>> + Valid values are 0-15, default is 1.
>> + - nvidia,snor-ce-width: Number of cycles before CE is asserted.
>> + Valid values are 0-255, default is 4
>
> ce-width only occupies a 4-bit field and so the max is 15.

ACK.

>
>> + - nvidia,snor-we-width: Number of cycles during which WE stays asserted.
>> + Valid values are 0-15, default is 1
>> + - nvidia,snor-oe-width: Number of cycles during which OE stays asserted.
>> + Valid values are 0-255, default is 1
>> + - nvidia,snor-wait-width: Number of cycles before READY is asserted.
>> + Valid values are 0-255, default is 3
>> +
>> +Example with two SJA1000 CAN controllers connected to the GMI bus:
>> +
>> + gmi@70090000 {
>> + #address-cells = <1>;
>> + #size-cells = <1>;
>
> I think 0 for size makes sense. I know that caused you problems before,
> but I am wondering if ...
>
>> + ranges;
>
> ... ranges is needed here? If we do have it, I am wondering if it should
> be a single entry for the chip-select that is being used. For now we
> could only support a ranges with one entry.
>
> #address-cells = <1>;
> #size-cells = <1>;
> ranges = <4 0x48000000 0x00040000>;

I prefer if we have "ranges" with one single entry, and warn if user enters
multiple for now, like we discussed earlier. Should have really done it in
this series.

>
>> + nvidia,snor-mux-mode;
>> + nvidia,snor-adv-inv;
>> + nvidia,snor-cs-select = <4>;
>
> I would have expected these under bus@X node as they are specific to the
> GMI CS.

Yes, that is true.

>
> I would also expect that the actual chip-select number is encoded in the
> reg property.
>
>> +
>> + bus@0,0 {
>
> bus@4

ACK.

>
> No mention of this bus node in the above documentation.

I was hesitant documenting it since I am not sure if we really need it
in a generic case? It does make sense in my
specific case. But what would it look like if we could maintain CS in software.

Do you then have a bus node per CS? I am guessing not. Is it enough to
document it in my "example brief"?

I should probably extend the example with some more configurations,
e.g. only one child node.

What I am thinking is something like

Example with two SJA1000 CAN controllers connected to the GMI bus.
We wrap the controllers with a simple-bus node since they are
all connected to the same chip-select (CS4) but external address decoding is
provided in hardware:

gmi@70090000 {
#address-cells = <1>;
#size-cells = <1>;
ranges = <4 0x48000000 0x00040000>;

bus@4 {
compatible = "simple-bus";
reg = <4>;

#address-cells = <1>;
#size-cells = <1>;

nvidia,snor-mux-mode;
nvidia,snor-adv-inv;

can@0 {
reg = <0 0x100>;
...
};

can@40000 {
reg = <0x40000 0x100>;
...
};
};
};

Example with one SJA1000 CAN controller connected to the GMI bus
on CS4:

gmi@70090000 {
#address-cells = <1>;
#size-cells = <1>;
ranges = <4 0x48000000 0x00040000>;

can@4 {
nvidia,snor-mux-mode;
nvidia,snor-adv-inv;
reg = <4 0x100>;
...
};
};

>
>> + compatible = "simple-bus";
>> + reg = <0 0>;
>
> reg = <4>;
>
> We should look up the chip-select from the reg property.

ACK.
>
>> + ranges;
>> +
>> + #address-cells = <1>;
>> + #size-cells = <1>;
>> +
>> + can@48000000 {
>> + reg = <0x48000000 0x100>;
>> + ...
>> + };
>> +
>> + can@48040000 {
>> + reg = <0x48040000 0x100>;
>> + ...
>> + };
>
> If we use ranges we could have ...
>
> can@0 {
> reg = <0x0 0x100>;
> ...
> };
>
> can@40000 {
> reg = <0x40000 0x100>;
> ...
> };
>
> Nit ... please use tabs for spacing as we do in the dtsi/dts files.

ACK.

Best Regards
Mirza Krak