Issue
In my opinion, SPI and DMA are both controllers.
SPI is a communication tool and DMA can transfer data without CPU.
The system API such as spi_sync() or spi_async(), are controlled by the CPU.
So what is the meaning of SPI with DMA, does it mean DMA can control the SPI API without CPU? Or the SPI control uses CPU but the data transfer to DMA directly?
Solution
SPI is not a tool, it is a communication protocol. Typical micro controllers have that protocol implemented in hardware which can accessed by read/write to dedicated registers in the address space of the given controller.
DMA on micro controllers is typically designed to move content of registers to memory and visa versa. DMA can sometimes configured to write a special amount of read/writes or increasing or decreasing source and target address of memory and so on.
If you have a micro controller which have SPI with DMA support, it typically means that you can have some data in the memory which will be transferred to the SPI unit to send multiple data bytes without intervention of the cpu core itself. Or read an amount of data bytes from SPI to memory automatically without wasting cpu core.
How such DMA SPI transfers are configured is written in the data sheets of the controllers. There are a very wide span of types so no specific information can be given here without knowing the micro type.
The linux APIs for dealing with SPI are abstracting the access of DMA and SPI by using the micro controller specific implementations in the drivers.
It is quite unclear if you want to use the API to access your SPI or you want to implement a device driver to make the linux API working on your specific controller.
It is not possible to give you a general introduction to write a kernel driver here or clarify register by register from your data sheets. If you need further information you have to make your question much more specific!
Answered By – Klaus
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