Re: [PATCH 3/5] PCI: rcar: Add R-Car PCIe endpoint device tree bindings

[Kishon Vijay Abraham I <kishon@ti.com>]

Hi,

On 13/11/19 9:38 AM, Rob Herring wrote:
> On Thu, Nov 07, 2019 at 09:08:35PM +0100, Geert Uytterhoeven wrote:
>> Hi Prabhakar,
>>
>> On Thu, Nov 7, 2019 at 10:26 AM Lad, Prabhakar
>> <prabhakar.csengg@gmail.com> wrote:
>>> On Thu, Nov 7, 2019 at 8:44 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>>>> On Wed, Nov 6, 2019 at 8:36 PM Lad Prabhakar <prabhakar.csengg@gmail.com> wrote:
>>>>> From: "Lad, Prabhakar" <prabhakar.mahadev-lad.rj@bp.renesas.com>
>>>>>
>>>>> This patch adds the bindings for the R-Car PCIe endpoint driver.
>>>>>
>>>>> Signed-off-by: Lad, Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
>>>>
>>>> Thanks for your patch!
>>>>
>>>>> --- /dev/null
>>>>> +++ b/Documentation/devicetree/bindings/pci/rcar-pci-ep.txt
>>>>> @@ -0,0 +1,43 @@
>>>>> +* Renesas R-Car PCIe Endpoint Controller DT description
>>>>> +
>>>>> +Required properties:
>>>>> +           "renesas,pcie-ep-r8a774c0" for the R8A774C0 SoC;
>>>>> +           "renesas,pcie-ep-rcar-gen3" for a generic R-Car Gen3 or
>>>>> +                                    RZ/G2 compatible device.
>>>>
>>>> Unless I'm missing something, this is for the exact same hardware block as
>>>> Documentation/devicetree/bindings/pci/rcar-pci.txt?
>>>> So shouldn't you amend those bindings, instead of adding new compatible
>>>> values?
>>>> Please remember that DT describes hardware, not software policy.
>>>> So IMHO choosing between host and endpoint is purely a configuration
>>>> issue, and could be indicated by the presence or lack of some DT properties.
>>>> E.g. host mode requires both "bus-range" and "device_type" properties,
>>>> so their absence could indicate endpoint mode.
>>>>
>>> yes its the same hardware block as described in the rcar-pci.txt, I
>>> did think about amending it
>>> but  it might turn out to be bit messy,
>>>
>>> required properties host ======required properties Endpoint
>>> ====================||==================
>>> 1: reg                                || reg
>>> 2:bus-range                      || reg names
>>> 3: device_type                  || resets
>>> 4: ranges                          || clocks
>>> 5: dma-ranges                  || clock-names
>>> 6: interrupts                      ||
>>> 7: interrupt-cells               ||
>>> 8: interrupt-map-mask     ||
>>> 9: clocks                          ||
>>> 10: clock-names             ||
>>
>> We have a similar situation with SPI, where a controller can operate in
>> master or slave mode, based on the absence or presence of the
>> "spi-slave" DT property.
>>
>>> and if I go ahead with the same compatible string that would mean to
>>> add support for endpoint
>>> mode in the host driver itself. I did follow the examples of
>>
>> You can still have two separate drivers, binding against the same
>> compatible value.  Just let the .probe() function return -ENODEV if it
>> discovers (by looking at DT properties) if the node is configured for
>> the other mode.
>> Which brings us to my next questions: is there any code that could be
>> shared between the drivers for the two modes?
>>
>>> rockchip/cadence/designware where
>>> its the same hardware block but has two different binding files one
>>> for host mode and other for
>>> endpoint mode.
>>
>> Having two separate DT binding documents sounds fine to me, if unifying
>> them makes things too complex.
>> However, I think they should use the same compatible value, because the
>> hardware block is the same, but just used in a different mode.
>>
>> Rob/Mark: Any input from the DT maintainers?
> 
> Separate files makes sense because different modes will want to 
> include different common schemas. We've generally been doing different 
> compatibles too which makes validating the node has the right set of 
> properties easier.
>  
>>>>> +- reg: Five register ranges as listed in the reg-names property
>>>>> +- reg-names: Must include the following names
>>>>> +       - "apb-base"
>>>>> +       - "memory0"
>>>>> +       - "memory1"
>>>>> +       - "memory2"
>>>>> +       - "memory3"
>>>>
>>>> What is the purpose of the last 4 regions?
>>>> Can they be chosen by the driver, at runtime?
>>>>
>>> no the driver cannot choose them at runtime, as these are the only
>>> PCIE memory(0/1/2/3) ranges
>>> in the AXI address space where host memory can be mapped.
>>
>> Are they fixed by the PCIe hardware, i.e. could they be looked up by the
>> driver based on the compatible value?
> 
> That would be strange for a memory range.
> 
> Sounds like like 'ranges' though I'm not sure if 'ranges' for an EP 
> makes sense or what that should look like.

These are similar to "memory node" with multiple address, size pairs. I'm
thinking if these should be added as a subnode within PCIe EP controller device
tree node?

Thanks
Kishon