Re: Single SPI Protocol Driver with multiple SPI Slaves

[Lars-Peter Clausen <lars@xxxxxxxxxx>]

On 09/05/2017 11:13 AM, Léo Serre wrote:
> Hello,
> 
> I have written three SPI drivers for BMX055 sensor (accelerometer +
> gyroscope + magnetometer). It appears that in that case, it was perfect to
> write three different drivers as the different components were independants.
> 
> I'm now working on a driver for MRF89XA (Microchip's radio chip) that is
> composed of two SPI slaves (one for config and one for data). The device is
> attached to the driver using a Device Tree.
> 
> Is that possible to write a single driver, that works with a pair of SPI
> slaves?

There are currently no facilities in the kernel to do this. I've have had a
similar problem with a different driver and given this some thought.

Conceptually I think the best approach is to have something like
i2c_new_dummy() that creates a second slave device with a different address
for an existing device.

The main issue with implementing this is the global spi_add_lock mutex which
prevents a SPI device from being created from within the probe() callback of
another SPI device.

The lock is used to prevent two SPI devices with the same chip-select being
registered on the same master concurrently and overwriting each others
configuration.

One way to remove the need for the lock could be to use a per master atomic
bitmap that keeps track of the chip-selects that are in use. But this would
require some additional memory in the spi_master struct.

- Lars
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