linux-kernel - Re: [PATCH] spi: Add spi-bits-per-word binding.

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Date:   Mon, 13 Mar 2017 20:57:36 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Adrian Fiergolski <Adrian.Fiergolski@...n.ch>
Cc:     Mark Brown <broonie@...nel.org>,
        linux-spi <linux-spi@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Miguel Ojeda Sandonis <miguel.ojeda.sandonis@...il.com>
Subject: Re: [PATCH] spi: Add spi-bits-per-word binding.

Hi Adrian,

On Mon, Mar 13, 2017 at 7:12 PM, Adrian Fiergolski
<Adrian.Fiergolski@...n.ch> wrote:
> On 13.03.2017 at 18:55, Mark Brown wrote:
>>> In my case, xilinx_spi_probe function (of spi-xilinx controller) sets
>>> bits_per_word_mask of spi_master struct only to 16 bits support. Later,
>>> xilinx_spi_probe calls of_register_spi_devices, which calls
>>> of_register_spi_devices. The last one allocates an empty spi_device
>>> struct and configures different options of the spi_device according to a
>>> device tree. bits_per_word are not covered here (why?), thus it is left
>>> 0 (value after allocation), which, by convention, means 8 bits support.
>>> At the end, the same function (of_register_spi_device) calls
>>> spi_add_device which finally calls spi_setup. The last call, according
>>> to convention, changes bits_per_word to 8 and calls
>>> __spi_validate_bits_per_word which fails, as master doesn't support 8
>>> bit transmission. This fails registration sequence of a device driver.
>>> As you see, the device driver doesn't have possibility to modify
>>> bits_per_word during the registration process, thus it can't provide
>>> support for such limited controllers.
>> I can't see any way in which it follows from the above that it's a good
>> idea to try to override bits per word settings in the device tree, that
>> just wastes user time and is an abstraction failure.  We need better
>> handling of defaults done purely in the kernel.
> If enforcing by device tree specific for a given device driver SPI_CPHA,
> SPIC_CPOL, SPI_CS_HIGH, max_speed_hz, etc. if fine form the abstraction
> point of view, why it doesn't apply to bits_per_word ?

Because unlike polarity, phase, and speed, bits_per_word is a property
of the communication protocol.

E.g. you can talk to the same EEPROM using different polarities, phase, or
speed, but bits_per_word is fixed.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds