spi: dw: Support DMA with a single channel

Some platforms, such as StarFive JHB100, provide only one DMA channel
for the DW SPI controller. dw_spi_dma_init_generic() requires both the
Tx and Rx channels, so DMA is disabled entirely on such platforms.

Request both channels at probe time and decide what is usable from the
return values. If the first (Rx) channel request fails, fall back to
non-DMA transfers as before. If only the second (Tx) channel request
fails, keep the Rx channel. Each successfully requested channel is
counted in dws->dma_nr_chans. -EPROBE_DEFER on the second channel is
still propagated.

Full-duplex standard SPI transfers need both channels, so only set
ctlr->can_dma and SPI_CONTROLLER_MUST_TX when two channels are
available. A single channel can still serve half-duplex transfers,
which will be used by the enhanced SPI memory operations in a
follow-up patch.

Make dw_spi_dma_caps_init() and dw_spi_dma_maxburst_init() cope with a
missing channel, and set ctlr->dma_tx/dma_rx only after the capability
check has passed so that they never point to a released channel.

Signed-off-by: Changhuang Liang <changhuang.liang@starfivetech.com>
Link: https://patch.msgid.link/20260930071535.7947-3-changhuang.liang@starfivetech.com
Signed-off-by: Mark Brown <broonie@kernel.org>

0 files changed, 0 insertions, 0 deletions