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Add SPI offload support to enable DMA-based, CPU-independent data acquisition using the SPI Engine offload framework. When an SPI offload is available (devm_spi_offload_get() succeeds), the driver registers a DMA engine IIO buffer and uses dedicated buffer setup operations. If no offload is available the existing software triggered buffer path is used unchanged. Both CNV Burst Mode and Manual Mode support offload, but use different trigger mechanisms: CNV Burst Mode: the SPI Engine is triggered by the ADC's DATA_READY signal on the GP pin specified by the trigger-source consumer reference in the device tree (one cell = GP pin number 0-3). For this mode the driver acts as both an SPI offload consumer (DMA RX stream, message optimization) and a trigger source provider: it registers the GP/DATA_READY output via devm_spi_offload_trigger_register() so the offload framework can match the '#trigger-source-cells' phandle and automatically fire the SPI Engine DMA transfer at end-of-conversion. Manual Mode: the SPI Engine is triggered by a periodic trigger at the configured sampling frequency. The pre-built SPI message uses the pipelined CNV-on-CS protocol: N+1 16-bit transfers are issued for N active channels (the first result is discarded as garbage from the pipeline flush) and the remaining N results are captured by DMA. All offload transfers use 16-bit frames (bits_per_word=16, len=2). The SPI Engine assembles received bits into native 16-bit words before DMA, so offload samples land in CPU-native byte order (IIO_CPU). Dedicated channel arrays (AD4691_OFFLOAD_CHANNEL) reflect this: they omit IIO_BE and carry no soft timestamp (DMA delivers data directly to userspace). The software triggered-buffer path retains its IIO_BE channels because bits_per_word=8 causes SPI to deliver bytes MSB-first into memory, making the on-disk layout big-endian. Both paths use storagebits=16 as transfers are 16 bits wide in both cases. IIO_BUFFER_DMAENGINE is selected because the offload path uses devm_iio_dmaengine_buffer_setup_with_handle() to allocate and attach the DMA RX buffer to the IIO device. Signed-off-by: Radu Sabau <radu.sabau@analog.com> Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. 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