dmaengine: Add support for repeating transactions

DMA engines used with displays perform 2D interleaved transfers to read
framebuffers from memory and feed the data to the display engine. As the
same framebuffer can be displayed for multiple frames, the DMA
transactions need to be repeated until a new framebuffer replaces the
current one. This feature is implemented natively by some DMA engines
that have the ability to repeat transactions and switch to a new
transaction at the end of a transfer without any race condition or frame
loss.

This patch implements support for this feature in the DMA engine API. A
new DMA_PREP_REPEAT transaction flag allows DMA clients to instruct the
DMA channel to repeat the transaction automatically until one or more
new transactions are issued on the channel (or until all active DMA
transfers are explicitly terminated with the dmaengine_terminate_*()
functions). A new DMA_REPEAT transaction type is also added for DMA
engine drivers to report their support of the DMA_PREP_REPEAT flag.

A new DMA_PREP_LOAD_EOT transaction flag is also introduced (with a
corresponding DMA_LOAD_EOT capability bit), as requested during the
review of v4. The flag instructs the DMA channel that the transaction
being queued should replace the active repeated transaction when the
latter terminates (at End Of Transaction). Not setting the flag will
result in the active repeated transaction to continue being repeated,
and the new transaction being silently ignored.

The DMA_PREP_REPEAT flag is currently supported for interleaved
transactions only. Its usage can easily be extended to cover more
transaction types simply by adding an appropriate check in the
corresponding dmaengine_prep_*() function.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20200717013337.24122-3-laurent.pinchart@ideasonboard.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
index e1c03339918f..328e3aca7f51 100644
--- a/include/linux/dmaengine.h
+++ b/include/linux/dmaengine.h
@@ -61,6 +61,8 @@ enum dma_transaction_type {
 	DMA_SLAVE,
 	DMA_CYCLIC,
 	DMA_INTERLEAVE,
+	DMA_REPEAT,
+	DMA_LOAD_EOT,
 /* last transaction type for creation of the capabilities mask */
 	DMA_TX_TYPE_END,
 };
@@ -176,6 +178,16 @@ struct dma_interleaved_template {
  * @DMA_PREP_CMD: tell the driver that the data passed to DMA API is command
  *  data and the descriptor should be in different format from normal
  *  data descriptors.
+ * @DMA_PREP_REPEAT: tell the driver that the transaction shall be automatically
+ *  repeated when it ends until a transaction is issued on the same channel
+ *  with the DMA_PREP_LOAD_EOT flag set. This flag is only applicable to
+ *  interleaved transactions and is ignored for all other transaction types.
+ * @DMA_PREP_LOAD_EOT: tell the driver that the transaction shall replace any
+ *  active repeated (as indicated by DMA_PREP_REPEAT) transaction when the
+ *  repeated transaction ends. Not setting this flag when the previously queued
+ *  transaction is marked with DMA_PREP_REPEAT will cause the new transaction
+ *  to never be processed and stay in the issued queue forever. The flag is
+ *  ignored if the previous transaction is not a repeated transaction.
  */
 enum dma_ctrl_flags {
 	DMA_PREP_INTERRUPT = (1 << 0),
@@ -186,6 +198,8 @@ enum dma_ctrl_flags {
 	DMA_PREP_FENCE = (1 << 5),
 	DMA_CTRL_REUSE = (1 << 6),
 	DMA_PREP_CMD = (1 << 7),
+	DMA_PREP_REPEAT = (1 << 8),
+	DMA_PREP_LOAD_EOT = (1 << 9),
 };
 
 /**
@@ -980,6 +994,9 @@ static inline struct dma_async_tx_descriptor *dmaengine_prep_interleaved_dma(
 {
 	if (!chan || !chan->device || !chan->device->device_prep_interleaved_dma)
 		return NULL;
+	if (flags & DMA_PREP_REPEAT &&
+	    !test_bit(DMA_REPEAT, chan->device->cap_mask.bits))
+		return NULL;
 
 	return chan->device->device_prep_interleaved_dma(chan, xt, flags);
 }