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

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, Oct 29, 2024 at 10:22:36AM -0400, Jim Quinlan wrote:
> On Tue, Oct 29, 2024 at 1:17 AM Krishna Chaitanya Chundru
> <quic_krichai@quicinc.com> wrote:
> > On 10/29/2024 12:21 AM, James Quinlan wrote:
> > > 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@broadcom.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).
> > >
> > I submitted a pull request for this. if you have any other suggestions
> > or if we need to have any other details we can update this pull request.
> > https://github.com/devicetree-org/dt-schema/pull/146
> 
> Thanks for doing this.   However, why a u8 array?  The registers are
> defined as u16 so it would be more natural to use a u16 array, each
> entry giving
> all of the data for a single lane.  In our implementation we read a
> u16 and we write it to the register -- what advantage is there by
> using a u8 array?
> 
> Also if there are 16 lanes, you will need 32 maxItems, correct?

I added these questions to the github PR.

Also, why does it define gen3-eq-presets, gen4-eq-presets,
gen5-eq-presets, and gen6-eq-presets?  I think there's only a single
place to write these values (the Lane Equalization Control registers,
PCIe r6.0, sec 7.7.3.4), isn't here?  How would software choose the
correct values to use?

> > >> - 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
> > >>>
> > >