linux-kernel - Re: [PATCH 1/1] RFC: dt bindings: Add property "brcm,gen3-eq-presets"

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Message-ID: <954d6c11-ab4e-485f-8152-94bf38625f9c@broadcom.com>
Date: Mon, 28 Oct 2024 14:51:34 -0400
From: James Quinlan <james.quinlan@...adcom.com>
To: Krishna Chaitanya Chundru <quic_krichai@...cinc.com>,
 Rob Herring <robh@...nel.org>
Cc: linux-pci@...r.kernel.org, Nicolas Saenz Julienne <nsaenz@...nel.org>,
 Bjorn Helgaas <bhelgaas@...gle.com>,
 Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
 bcm-kernel-feedback-list@...adcom.com, jim2101024@...il.com,
 Florian Fainelli <florian.fainelli@...adcom.com>,
 Lorenzo Pieralisi <lpieralisi@...nel.org>,
 Krzysztof Wilczyński <kw@...ux.com>,
 Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>,
 "moderated list:BROADCOM BCM7XXX ARM ARCHITECTURE"
 <linux-arm-kernel@...ts.infradead.org>,
 "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE"
 <linux-rpi-kernel@...ts.infradead.org>,
 "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS"
 <devicetree@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] RFC: dt bindings: Add property "brcm,gen3-eq-presets"

On 10/24/24 21:08, Krishna Chaitanya Chundru wrote:
>
>
> On 10/22/2024 12:33 AM, Rob Herring wrote:
>> On Fri, Oct 18, 2024 at 02:22:45PM -0400, Jim Quinlan wrote:
>>> Support configuration of the GEN3 preset equalization settings, aka the
>>> Lane Equalization Control Register(s) of the Secondary PCI Express
>>> Extended Capability.  These registers are of type HwInit/RsvdP and
>>> typically set by FW.  In our case they are set by our RC host bridge
>>> driver using internal registers.
>>>
>>> Signed-off-by: Jim Quinlan <james.quinlan@...adcom.com>
>>> ---
>>>   .../devicetree/bindings/pci/brcm,stb-pcie.yaml       | 12 
>>> ++++++++++++
>>>   1 file changed, 12 insertions(+)
>>>
>>> diff --git 
>>> a/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml 
>>> b/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml
>>> index 0925c520195a..f965ad57f32f 100644
>>> --- a/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml
>>> +++ b/Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml
>>> @@ -104,6 +104,18 @@ properties:
>>>       minItems: 1
>>>       maxItems: 3
>>>   +  brcm,gen3-eq-presets:
>>> +    description: |
>>> +      A u16 array giving the GEN3 equilization presets, one for 
>>> each lane.
>>> +      These values are destined for the 16bit registers known as the
>>> +      Lane Equalization Control Register(s) of the Secondary PCI 
>>> Express
>>> +      Extended Capability.  In the array, lane 0 is first term, 
>>> lane 1 next,
>>> +      etc. The contents of the entries reflect what is necessary for
>>> +      the current board and SoC, and the details of each preset are
>>> +      described in Section 7.27.4 of the PCI base spec, Revision 3.0.
>>
>> If these are defined by the PCIe spec, then why is it Broadcom specific
>> property?
Yes, I will remove the "brcm," prefix.
>>
> Hi Rob,
>
> qcom pcie driver also needs to program these presets as you suggested
> this can go to common pci bridge binding.
>
> from PCIe spec 6.0.1 revision section 8.3.3.3 & 4.2.4.2 for data rates
> of  8.0 GT/s, 16.0 GT/s, and 32.0 GT/s uses one class of preset (P0
> through P10) and where as data rates of 64.0 GT/s use different class of
> presets (Q0 through Q10) (Table 4-23). And data rates of 8.0 GT/s also
> have optional preset hints (Table 4-24).
>
> And there is possibility that for each data rate we may require
> different preset configuration.
>
> Can we have a dt binding for each data rate of 16 byte array.
> like gen3-eq-preset array, gen4-eq-preset array etc.

Yes, that was the idea when using "genX-eq-preset", for X in {3,4...}.

Keep in mind that this is an RFC; I have a backlog of commit submissions 
before I can submit the code that uses this DT property.  If you 
(Krishna) want to submit something now I'd be quite happy to go with 
that.  I don't believe it is acceptable to submit a bindings commit w/o 
code that uses it (if I'm incorrect I'll be glad to do a V2).

Regards,

Jim Quinlan
Broadcom STB/CM

>
> - Krishna Chaitanya
>>> +
>>> +    $ref: /schemas/types.yaml#/definitions/uint16-array
>>
>> minItems: 1
>> maxItems: 16
>>
>> Last I saw, you can only have up to 16 lanes.
>>
>> Rob
>>