linux-kernel - Re: [PATCH] spi: spi-geni-qcom: Add SPI driver support for GENI based QUP

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Message-ID: <20180810105205.GC20971@sirena.org.uk>
Date:   Fri, 10 Aug 2018 11:52:05 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Dilip Kota <dkota@...eaurora.org>,
        Stephen Boyd <swboyd@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-spi <linux-spi@...r.kernel.org>,
        Sagar Dharia <sdharia@...eaurora.org>,
        Karthikeyan Ramasubramanian <kramasub@...eaurora.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "Mahadevan, Girish" <girishm@...eaurora.org>
Subject: Re: [PATCH] spi: spi-geni-qcom: Add SPI driver support for GENI
 based QUP

On Thu, Aug 09, 2018 at 11:03:55AM -0700, Doug Anderson wrote:
> On Fri, Aug 3, 2018 at 5:18 AM,  <dkota@...eaurora.org> wrote:

> > Also, spi core framework will set the transfer speed to controller max
> > frequency
> > if transfer frequency is greater than controller max frequency.
> > Please mention if you have a other opinion.

> 1. It sure seems like the clock framework could be enforcing the max
> speed here.  SPI can just ask for the speed and the clock framework
> will pick the highest speed it can if you ask for one too high.  Isn't
> that the whole point of the "struct freq_tbl" in the clock driver?

This is more about matching the data rate between the two drivers - the
clock framework could (and possibly should) reasonably return an error
here, we're trying to ensure that drivers and controllers work well
together here.

> 2. The device tree writer already provides a max clock speed for each
> SPI slave in the device tree.  ...shouldn't the device tree writer
> already be taking into account the max of the SPI port when setting
> this value?

Yes.  We're overriding this because drivers can set a speed from code
(this is especially common when devices have variable maximum speeds for
different operations).

> 3. If you really truly need code in the SPI driver then make sure you
> include a compatible string for the SoC and have a table in the driver
> that's found with of_device_get_match_data().  AKA:

>   compatible = "qcom,geni-spi-sdm845", "qcom,geni-spi";

A controller driver really shouldn't need to be open coding anything.

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