linux-kernel - Re: [linux-sunxi] [PATCH] spi: dts: sun4i: Add support for inter-word wait cycles using the SPI Wait Clock Register

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Message-ID: <CAGRGNgV0kOkye-Zu4WGQKgGim7N2HB9rAFJ3HNQS89Rr6_WexQ@mail.gmail.com>
Date:	Fri, 20 Nov 2015 21:12:04 +1100
From:	Julian Calaby <julian.calaby@...il.com>
To:	Marcus Weseloh <mweseloh42@...il.com>
Cc:	linux-sunxi <linux-sunxi@...glegroups.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Mark Brown <broonie@...nel.org>,
	devicetree <devicetree@...r.kernel.org>,
	"Mailing List, Arm" <linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-spi@...r.kernel.org
Subject: Re: [linux-sunxi] [PATCH] spi: dts: sun4i: Add support for inter-word
 wait cycles using the SPI Wait Clock Register

Hi Marcus,

On Fri, Nov 20, 2015 at 7:45 PM, Marcus Weseloh <mweseloh42@...il.com> wrote:
> Hi Julian,
>
> 2015-11-19 23:59 GMT+01:00 Julian Calaby <julian.calaby@...il.com>:
>> Should you possibly hide the 3 clock periods from the user?
>>
>> I.e. they set whatever they want for the wdelay, we set it to the
>> closest number we can that's greater or equal to what they ask for.
>
> That's a good idea and much better than having to remember to subtract
> 3 cycles from the desired wait time!
>
> But it would mean that this magic number becomes part of the driver
> code. I have found no official documentation that mentions those
> additional cycles. While I have checked many different transmission
> speeds using both CDR1 and CDR2 divider configurations, there is still
> the possibility that the behaviour changes with weird SPI module
> configurations... And I've only tested it on A20 hardware. So it would
> be great if somebody else with access to A10 hardware and an
> oscilloscope could check if we have a consistent 3 cycle overhead.

Having magic numbers is kind-of a drivers' job. (and the wdelay should
arguably be a core-spi thing, not a sunxi thing, but that's a separate
discussion)

If it is different for other SoCs, then you might have to move that
constant somewhere else and introduce new compatible strings for them.

Thanks,

-- 
Julian Calaby

Email: julian.calaby@...il.com
Profile: http://www.google.com/profiles/julian.calaby/
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