Hi. When I wrote my DMA engine driver, I had difficulty in understanding what to set to the struct dma_device members. For example, src_addr_widths. I see two driver groups regarding this. [A] BIT(DMA_SLAVE_BUSWIDTH_*_BYTES) are OR'ed For example, st_fdma.c https://github.com/torvalds/linux/blob/v4.19-rc4/drivers/dma/st_fdma.c#L729 [B] DMA_SLAVE_BUSWIDTH_*_BYTES are directly OR'ed without BIT() For example, sh/rcar-dmac.c https://github.com/torvalds/linux/blob/v4.19-rc4/drivers/dma/sh/rcar-dmac.c#L1778 Which is correct? If we go with [A], BIT(DMA_SLAVE_BUSWIDTH_64_BYTES) i.e. BIT(64) would not fit in ddevice->src_addr_widths whose type is u32. If we go with [B], the following formula is met. DMA_SLAVE_BUSWIDTH_3_BYTES == DMA_SLAVE_BUSWIDTH_1_BYTE | DMA_SLAVE_BUSWIDTH_2_BYTES We never know whether 3_BYTES is supported, or it is just a combination of 1_BYTE and 2_BYTES. -- Best Regards Masahiro Yamada
