dmaengine: Introduce max SG burst capability

Some devices may lack the support of the hardware accelerated SG list
entries automatic walking through and execution. In this case a burden of
the SG list traversal and DMA engine re-initialization lies on the
DMA engine driver (normally implemented by using a DMA transfer completion
IRQ to recharge the DMA device with a next SG list entry). But such
solution may not be suitable for some DMA consumers. In particular SPI
devices need both Tx and Rx DMA channels work synchronously in order
to avoid the Rx FIFO overflow. In case if Rx DMA channel is paused for
some time while the Tx DMA channel works implicitly pulling data into the
Rx FIFO, the later will be eventually overflown, which will cause the data
loss. So if SG list entries aren't automatically fetched by the DMA
engine, but are one-by-one manually selected for execution in the
ISRs/deferred work/etc., such problem will eventually happen due to the
non-deterministic latencies of the service execution.

In order to let the DMA consumer know about the DMA device capabilities
regarding the hardware accelerated SG list traversal we introduce the
max_sg_burst capability. It is supposed to be initialized by the DMA engine
driver with 0 if there is no limitation of the number of SG entries
atomically executed and with non-zero value if there is such constraints,
so the upper limit is determined by the number set to the property.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200723005848.31907-5-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Vinod Koul <vkoul@kernel.org>

2 files changed, 9 insertions, 0 deletions

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index 2f1a7c0c5446..8177f78faeda 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -594,6 +594,7 @@ int dma_get_slave_caps(struct dma_chan *chan, struct dma_slave_caps *caps)
 	caps->directions = device->directions;
 	caps->min_burst = device->min_burst;
 	caps->max_burst = device->max_burst;
+	caps->max_sg_burst = device->max_sg_burst;
 	caps->residue_granularity = device->residue_granularity;
 	caps->descriptor_reuse = device->descriptor_reuse;
 	caps->cmd_pause = !!device->device_pause;
diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
index 7d6e2aa26980..4cbe09e66db2 100644
--- a/include/linux/dmaengine.h
+++ b/include/linux/dmaengine.h
@@ -469,6 +469,9 @@ enum dma_residue_granularity {
  *	should be checked by controller as well
  * @min_burst: min burst capability per-transfer
  * @max_burst: max burst capability per-transfer
+ * @max_sg_burst: max number of SG list entries executed in a single burst
+ *	DMA tansaction with no software intervention for reinitialization.
+ *	Zero value means unlimited number of entries.
  * @cmd_pause: true, if pause is supported (i.e. for reading residue or
  *	       for resume later)
  * @cmd_resume: true, if resume is supported
@@ -483,6 +486,7 @@ struct dma_slave_caps {
 	u32 directions;
 	u32 min_burst;
 	u32 max_burst;
+	u32 max_sg_burst;
 	bool cmd_pause;
 	bool cmd_resume;
 	bool cmd_terminate;
@@ -775,6 +779,9 @@ struct dma_filter {
  *	should be checked by controller as well
  * @min_burst: min burst capability per-transfer
  * @max_burst: max burst capability per-transfer
+ * @max_sg_burst: max number of SG list entries executed in a single burst
+ *	DMA tansaction with no software intervention for reinitialization.
+ *	Zero value means unlimited number of entries.
  * @residue_granularity: granularity of the transfer residue reported
  *	by tx_status
  * @device_alloc_chan_resources: allocate resources and return the
@@ -846,6 +853,7 @@ struct dma_device {
 	u32 directions;
 	u32 min_burst;
 	u32 max_burst;
+	u32 max_sg_burst;
 	bool descriptor_reuse;
 	enum dma_residue_granularity residue_granularity;