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Subject: Re: [PATCH 05/10] dt-bindings: PCI: tegra: Add device tree support
 for T194
To:     Thierry Reding <thierry.reding@gmail.com>,
        Rob Herring <robh@kernel.org>
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From:   Vidya Sagar <vidyas@nvidia.com>
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On 4/2/2019 7:50 PM, Thierry Reding wrote:
> On Tue, Apr 02, 2019 at 02:46:27PM +0530, Vidya Sagar wrote:
>> On 4/1/2019 8:01 PM, Thierry Reding wrote:
>>> On Mon, Apr 01, 2019 at 04:48:42PM +0530, Vidya Sagar wrote:
>>>> On 3/31/2019 12:12 PM, Rob Herring wrote:
>>>>> On Tue, Mar 26, 2019 at 08:43:22PM +0530, Vidya Sagar wrote:
> [...]
>>>>>> +Optional properties:
>>>>>> +- nvidia,max-speed: limits controllers max speed to this value.
>>>>>> +    1 - Gen-1 (2.5 GT/s)
>>>>>> +    2 - Gen-2 (5 GT/s)
>>>>>> +    3 - Gen-3 (8 GT/s)
>>>>>> +    4 - Gen-4 (16 GT/s)
>>>>>> +- nvidia,init-speed: limits controllers init speed to this value.
>>>>>> +    1 - Gen-1 (2. 5 GT/s)
>>>>>> +    2 - Gen-2 (5 GT/s)
>>>>>> +    3 - Gen-3 (8 GT/s)
>>>>>> +    4 - Gen-4 (16 GT/s)
>>>>>
>>>>> Don't we have standard speed properties?
>>>> Not that I'm aware of. If you come across any, please do let me know and
>>>> I'll change these.
>>>
>>> See Documentation/devicetree/bindings/pci/pci.txt, max-link-speed.
>>> There's a standard of_pci_get_max_link_speed() property that reads it
>>> from device tree.
>> Thanks for the pointer. I'll switch to this in the next patch.
>>
>>>
>>>>> Why do we need 2 values?
>>>> max-speed configures the controller to advertise the speed mentioned through
>>>> this flag, whereas, init-speed gets the link up at this speed and software
>>>> can further take the link speed to a different speed by retraining the link.
>>>> This is to give flexibility to developers depending on the platform.
>>>
>>> This seems to me like overcomplicating things. Couldn't we do something
>>> like start in the slowest mode by default and then upgrade if endpoints
>>> support higher speeds?
>>>
>>> I'm assuming that the maximum speed is already fixed by the IP hardware
>>> instantiation, so why would we want to limit it additionally? Similarly,
>>> what's the use-case for setting the initial link speed to something
>>> other than the lowest speed?
>> You are right that maximum speed supported by hardware is fixed and through
>> max-link-speed DT option, flexibility is given to limit it to the desired speed
>> for a controller on a given platform. As mentioned in the documentation for max-link-speed,
>> this is a strategy to avoid unnecessary operation for unsupported link speed.
>> There is no real use-case as such even for setting the initial link speed, but it is
>> there to give flexibility (for any debugging) to get the link up at a certain speed
>> and then take it to a higher speed at a later point of time. Please note that, hardware
>> as such already has the capability to take the link to maximum speed agreed by both
>> upstream and downstream ports. 'nvidia,init-speed' is only to give more flexibility
>> while debugging. I'm OK to remove it if this is not adding much value here.
> 
> If this is primarily used for debugging or troubleshooting, maybe making
> it a module parameter is a better choice?
> 
> I can see how max-link-speed might be good in certain situations where a
> board layout may mandate that a link speed slower than the one supported
> by the hardware is used, but I can't imagine a case where the initial
> link speed would have to be limited based on the hardware designe.
> 
> Rob, do you know of any other cases where something like this is being
> used?
Fair enough. I'll make max-link-speed as an optional DT parameter and
leave 'nvidia,init-speed' to Rob to decided whether it is OK to have it
or it is not acceptable.

> 
>>>>>> +- nvidia,disable-aspm-states : controls advertisement of ASPM states
>>>>>> +    bit-0 to '1' : disables advertisement of ASPM-L0s
>>>>>> +    bit-1 to '1' : disables advertisement of ASPM-L1. This also disables
>>>>>> +                 advertisement of ASPM-L1.1 and ASPM-L1.2
>>>>>> +    bit-2 to '1' : disables advertisement of ASPM-L1.1
>>>>>> +    bit-3 to '1' : disables advertisement of ASPM-L1.2
>>>>>
>>>>> Seems like these too should be common.
>>>> This flag controls the advertisement of different ASPM states by root port.
>>>> Again, I'm not aware of any common method for this.
>>>
>>> rockchip-pcie-host.txt documents an "aspm-no-l0s" property that prevents
>>> the root complex from advertising L0s. Sounds like maybe following a
>>> similar scheme would be best for consistency. I think we'll also want
>>> these to be non-NVIDIA specific, so drop the "nvidia," prefix and maybe
>>> document them in pci.txt so that they can be more broadly used.
>> Since we have ASPM-L0s, L1, L1.1 and L1.2 states, I prefer to have just one entry
>> with different bit positions indicating which particular state should not be
>> advertised by root port. Do you see any particular advantage to have 4 different options?
>> If having one options is fine, I'll remove "nvidia," and document it in pci.txt.
> 
> I don't care strongly either way. It's really up to Rob to decide.
Rob, please let us know your comments on this also.

> 
> Thierry
>