Modify dw_edma_device_transfer() to also support the semantics of dma
device transfer for additional use cases involving pcitest utility as a
local initiator.

For its original use case, dw-edma supported the semantics of dma device
transfer from the perspective of a remote initiator who is located across
the PCIe bus from dma channel hardware.

To a remote initiator, DMA_DEV_TO_MEM means using a remote dma WRITE
channel to transfer from remote memory to local memory. A WRITE channel
would be employed on the remote device in order to move the contents of
remote memory to the bus destined for local memory.

To a remote initiator, DMA_MEM_TO_DEV means using a remote dma READ
channel to transfer from local memory to remote memory. A READ channel
would be employed on the remote device in order to move the contents of
local memory to the bus destined for remote memory.

>From the perspective of a local dma initiator who is co-located on the
same side of the PCIe bus as the dma channel hardware, the semantics of
dma device transfer are flipped.

To a local initiator, DMA_DEV_TO_MEM means using a local dma READ channel
to transfer from remote memory to local memory. A READ channel would be
employed on the local device in order to move the contents of remote
memory to the bus destined for local memory.

To a local initiator, DMA_MEM_TO_DEV means using a local dma WRITE channel
to transfer from local memory to remote memory. A WRITE channel would be
employed on the local device in order to move the contents of local memory
to the bus destined for remote memory.

To support local dma initiators, dw_edma_device_transfer() is modified to
now examine the direction field of struct dma_slave_config for the channel
which initiators can configure by calling dmaengine_slave_config().

If direction is configured as either DMA_DEV_TO_MEM or DMA_MEM_TO_DEV,
local initiator semantics are used. If direction is a value other than
DMA_DEV_TO_MEM nor DMA_MEM_TO_DEV, then remote initiator semantics are
used. This should maintain backward compatibility with the original use
case of dw-edma.

The dw-edma-test utility is an example of a remote initiator. From reading
its patch, dw-edma-test does not specifically set the direction field of
struct dma_slave_config. Since dw_edma_device_transfer() also does not
check the direction field of struct dma_slave_config, it seems safe to use
this convention in dw-edma to support both local and remote initiator
semantics.
Signed-off-by: Alan Mikhak <alan.mikhak@sifive.com>
Link: https://lore.kernel.org/r/1588122633-1552-1-git-send-email-alan.mikhak@sifive.comSigned-off-by: Vinod Koul <vkoul@kernel.org>