[RFC 3/6] dt/bindings: Add bindings for Tegra20/30 NOR bus driver

[Jon Hunter]

On 25/07/16 15:38, Mirza Krak wrote:
> 2016-07-25 16:15 GMT+02:00 Thierry Reding <thierry.reding at gmail.com>:
>> On Mon, Jul 25, 2016 at 03:16:28PM +0200, Mirza Krak wrote:
>>> 2016-07-25 13:30 GMT+02:00 Thierry Reding <thierry.reding at gmail.com>:
>> [...]
>>>> The above suggests that one of the CAN controllers gets mapped to an
>>>> address 0x48000000 and the other gets mapped to 0x48040000. But why do
>>>> we even need a chip-select at all in that case? If both chips don't use
>>>> any overlapping memory region, what good does the chip-select do?
>>>
>>> If we take a look on similar controllers found on others SOCs they
>>> usually define an address range / chip-select.
>>>
>>> Example (weim):
>>> ranges = <
>>> 0 0 0x10000000 0x02000000
>>> 1 0 0x12000000 0x01000000
>>> 2 0 0x13000000 0x01000000
>>> 3 0 0x14000000 0x01000000
>>> 4 0 0x15000000 0x01000000
>>> 5 0 0x16000000 0x01000000
>>>> ;
>>>
>>> Which means that you all ready have an address mapped to PIN function.
>>>
>>> But Tegra GMI controller is a first for me, where you do not have this
>>> kind of setup in hardware. Usually you also have a timing register /
>>> chip-select so that you can connect different chip types.
>>>
>>> The lack of hardware support do decode an address to a chip-select PIN
>>> function, we have implemented this our self externally.
>>>
>>> We actually have 6 different chips connected to the GMI bus and the
>>> "ranges" would be:
>>>   ranges = <
>>>    0 0 0x48000000 0x00000100
>>>    1 0 0x48040000 0x00000100
>>>    2 0 0x48080000 0x00000100
>>>    3 0 0x480A0000 0x00000100
>>>    4 0 0x480C0000 0x00000100
>>>    5 0 0x480E0000 0x00000100
>>>   >;
>>>
>>> And this not nothing complicated, small number of AND gates and that is it.
>>>
>>> The chip-select signal is necessary for the access characteristics, so
>>> we need to translate an address to an chip-select so that the chip
>>> knows the host CPU wants to talk to it.
>>>
>>> Do not know if I made anything more clear here :).
>>
>> Yes, that clarifies many things. The presence of an external, address-
>> based chip-select is essential information in order to describe this
>> setup properly.
>>
>> Given that the external chip select is entirely invisible to software, I
>> think a more accurate description of your setup would be:
>>
>>         gmi at 70090000 {
>>                 ...
>>
>>                 /* for the chip select */
>>                 #address-cells = <1>;
>>                 #size-cells = <0>;
>>
>>                 /*
>>                  * Technically this could be used to translate the range from
>>                  * 0x48000000 to 0x4fffffff into a different range, but that
>>                  * no longer works because of the #address-cells. Does this
>>                  * matter?
>>                  */
>>                 ranges;
>>
>>                 bus at 0 {
>>                         compatible = "simple-bus";
>>                         reg = <0>;
>>
>>                         #address-cells = <1>;
>>                         #size-cells = <1>;
>>
>>                         can at 48000000 {
>>                                 reg = <0x48000000 0x100>;
>>                                 ...
>>                         };
>>
>>                         can at 48040000 {
>>                                 reg = <0x48040000 0x100>;
>>                                 ...
>>                         };
>>                 };
>>         };
>>
>> That omits any reference to the external chip select, which I think
>> makes sense because it's something that software is completely unaware
>> of.
>>
>> Perhaps one important question: does your setup use the GMI's CS lines
>> in any way? Or does it simply get ignored?
> 
> Yes, we use the GMI`s CS line. It is important that is present because
> it is ANDED with address lines and NOT ALE line. Especially since we
> run the GMI controller in AD_MUX mode, which means that we use same
> pins for address and data and the access happens in two phases, one
> address latch phase where CS is not asserted but ALE is, second
> read/write phase where CS must be asserted. In this case we need the
> GMI`s CS line to determine which phase we are in.

Even though the CS is used, we could still implement the driver such
that if only 1 CS is defined in the binding, we then statically program
the configuration at probe. For now, if there is more than one, we can
return an error from probe as it is not supported or default to the
first CS defined and warn?

Jon

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