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

From: Geert Uytterhoeven
Date: Mon Mar 13 2017 - 15:57:49 EST

Hi Adrian,

On Mon, Mar 13, 2017 at 7:12 PM, Adrian Fiergolski
<Adrian.Fiergolski@xxxxxxx> 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.



Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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