linux-kernel - Re: [PATCH v3 5/8] dt-bindings: snps,dw-apb-ssi: Add sparx5 support, plus snps,rx-sample-delay-ns property

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Message-ID: <87zh82jy6c.fsf@soft-dev15.microsemi.net>
Date:   Tue, 14 Jul 2020 10:30:51 +0200
From:   Lars Povlsen <lars.povlsen@...rochip.com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        Mark Brown <broonie@...nel.org>
CC:     Rob Herring <robh@...nel.org>,
        Serge Semin <fancer.lancer@...il.com>,
        "Lars Povlsen" <lars.povlsen@...rochip.com>,
        Peter Rosin <peda@...ntia.se>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        <linux-spi@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 5/8] dt-bindings: snps,dw-apb-ssi: Add sparx5 support, plus snps,rx-sample-delay-ns property


Serge Semin writes:

> On Mon, Jul 13, 2020 at 01:22:59PM -0600, Rob Herring wrote:
>> On Thu, Jul 02, 2020 at 12:13:28PM +0200, Lars Povlsen wrote:
> ...
>>
>> > +
>> >  patternProperties:
>> >    "^.*@[0-9a-f]+$":
>> >      type: object
>> > @@ -107,6 +122,14 @@ patternProperties:
>> >        spi-tx-bus-width:
>> >          const: 1
>> >
>> > +      snps,rx-sample-delay-ns:
>>
>
>> We already have 'rx-sample-delay-ns' from Rockchip SPI, so use that. But
>> note that it applies to the SPI node. Does this need to be per SPI
>> child?
>
> It was me, who suggested to Lars to have that parameter moved to the SPI
> sub-nodes. As I see it the property is highly dependent on the SPI slave device
> the controller is communicating to. Some of the them may need the delay some
> may not. It's not always the capacitance thing, but also depends on how good
> the MISO signal a particular slave generates. So IMO the Rockchip SPI driver
> developer should have moved that property to the sub-nodes too.
>

In the case with sparx5, having two bus interfaces, spi slaves may be on
different busses - making it obvious why you may need different settings.

But I guess even on the same bus the physical length of SPI connections
and device response delay to each device could play in as well.

Nevertheless, it *does* make sense to be able to specify per slave, but
a default could make the DT more terse. Should I add this to my patch?

I will remove the "snps," prefix now that the property is globally
established.

---Lars

> On the other hand if the Rx errors are caused by the MISO lane capacitance,
> then it will be cumbersome to have the same property duplicated for each
> sub-node. Then what about having the property supported by both the SPI
> controller and the SPI-child nodes? For instance the SPI-controller
> "rx-sample-delay-ns" will provide a default sample delay for each sub-node
> instead of zero by default, while the individual sub-node "rx-sample-delay-ns"
> property can be used to override the default value.
>
> -Sergey
>
>>
>> BTW, the Rockchip controller appears to be a version of the DW
>> controller.
>>
>> > +        description: SPI Rx sample delay offset, unit is nanoseconds.
>> > +          The delay from the default sample time before the actual
>> > +          sample of the rxd input signal occurs. The "rx_sample_delay"
>> > +          is an optional feature of the designware controller, and the
>> > +          upper limit is also subject to controller configuration.
>> > +        $ref: /schemas/types.yaml#/definitions/uint32
>> > +
>> >  unevaluatedProperties: false
>> >
>> >  required:
>> > @@ -129,5 +152,10 @@ examples:
>> >        num-cs = <2>;
>> >        cs-gpios = <&gpio0 13 0>,
>> >                   <&gpio0 14 0>;
>> > +      spi-flash@1 {
>> > +        compatible = "spi-nand";
>> > +        reg = <1>;
>> > +        snps,rx-sample-delay-ns = <7>;
>> > +      };
>> >      };
>> >  ...
>> > --
>> > 2.27.0

-- 
Lars Povlsen,
Microchip