linux-kernel - Re: [PATCH v2 2/3] dt-bindings: PCI: dra7xx: Add properties to enable x2 lane in dra7

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Date:   Tue, 26 Dec 2017 11:46:37 -0600
From:   Rob Herring <robh@...nel.org>
To:     Tony Lindgren <tony@...mide.com>
Cc:     Kishon Vijay Abraham I <kishon@...com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        linux-omap <linux-omap@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, linux-pci@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Sekhar Nori <nsekhar@...com>
Subject: Re: [PATCH v2 2/3] dt-bindings: PCI: dra7xx: Add properties to enable
 x2 lane in dra7

On Fri, Dec 22, 2017 at 12:24 PM, Tony Lindgren <tony@...mide.com> wrote:
> * Kishon Vijay Abraham I <kishon@...com> [171222 06:06]:
>> Hi Rob,
>>
>> On Thursday 21 December 2017 12:27 AM, Rob Herring wrote:
>> > On Tue, Dec 19, 2017 at 02:28:22PM +0530, Kishon Vijay Abraham I wrote:
>> >> Add syscon properties required for configuring PCIe in x2 lane mode.
>> >>
>> >> Signed-off-by: Kishon Vijay Abraham I <kishon@...com>
>> >> Signed-off-by: Sekhar Nori <nsekhar@...com>
>> >> ---
>> >>  Documentation/devicetree/bindings/pci/ti-pci.txt | 6 ++++++
>> >>  1 file changed, 6 insertions(+)
>> >>
>> >> diff --git a/Documentation/devicetree/bindings/pci/ti-pci.txt b/Documentation/devicetree/bindings/pci/ti-pci.txt
>> >> index 82cb875e4cec..bfbc77ac7355 100644
>> >> --- a/Documentation/devicetree/bindings/pci/ti-pci.txt
>> >> +++ b/Documentation/devicetree/bindings/pci/ti-pci.txt
>> >> @@ -13,6 +13,12 @@ PCIe DesignWare Controller
>> >>   - ti,hwmods : Name of the hwmod associated to the pcie, "pcie<X>",
>> >>           where <X> is the instance number of the pcie from the HW spec.
>> >>   - num-lanes as specified in ../designware-pcie.txt
>> >> + - ti,syscon-lane-conf : phandle/offset pair. Phandle to the system control
>> >> +                   module and the register offset to specify 1 lane or
>> >> +                   2 lane.
>> >> + - ti,syscon-lane-sel : phandle/offset pair. Phandle to the system control
>> >> +                  module and the register offset to specify lane
>> >> +                  selection.
>> >
>> > Adding a property for every syscon register doesn't really scale and
>> > doesn't work if the register layout changes.
>>
>> The register layout doesn't really change between silicon revisions and for new
>> SoCs, the phandle and the register offset for that SoC will have to be
>> populated again.

And what about SoCs that don't exist yet?

>> Having said that, I'm not aware of any other alternative here.

What would you do if you had 20 different syscon registers to
configure? Add 20 properties? No, you would have per SoC functions in
the driver to handle the different cases.

> Sorry I did not realize this is still open. Sounds like I need to
> revert commit 4ece93c020e3 ("ARM: dts: dra7: Add properties to
> enable PCIe x2 lane mode"), let me know if that is not the case.

It's fine, I guess. Keep adding more syscon phandles and then I'll NAK
it (if I remember :)).

Rob