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

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Date:   Mon, 13 Mar 2017 19:12:13 +0100
From:   Adrian Fiergolski <Adrian.Fiergolski@...n.ch>
To:     Mark Brown <broonie@...nel.org>
CC:     <linux-spi@...r.kernel.org>, <robh+dt@...nel.org>,
        <mark.rutland@....com>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <miguel.ojeda.sandonis@...il.com>
Subject: Re: [PATCH] spi: Add spi-bits-per-word binding.

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 ?