[PATCH v3 10/10] dmaengine: dw: Initialize max_sg_nents with nollp flag

[Serge Semin]

Multi-block support provides a way to map the kernel-specific SG-table so
the DW DMA device would handle it as a whole instead of handling the
SG-list items or so called LLP block items one by one. So if true LLP
list isn't supported by the DW DMA engine, then soft-LLP mode will be
utilized to load and execute each LLP-block one by one. The soft-LLP mode
of the DMA transactions execution might not work well for some DMA
consumers like SPI due to its Tx and Rx buffers inter-dependency. Let's
expose the nollp flag indicating the soft-LLP mode by means of the
max_sg_nents capability, so the DMA consumer would be ready to somehow
workaround errors caused by such mode being utilized.

Signed-off-by: Serge Semin <Sergey.Semin@xxxxxxxxxxxxxxxxxxxx>
Cc: Alexey Malahov <Alexey.Malahov@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Rob Herring <robh+dt@xxxxxxxxxx>
Cc: linux-mips@xxxxxxxxxxxxxxx
Cc: devicetree@xxxxxxxxxxxxxxx

---

Changelog v3:
- This is a new patch created as a result of the discussion with Vinud and
  Andy in the framework of DW DMA burst and LLP capabilities.
---
 drivers/dma/dw/core.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/dma/dw/core.c b/drivers/dma/dw/core.c
index 29c4ef08311d..b850eb7fd084 100644
--- a/drivers/dma/dw/core.c
+++ b/drivers/dma/dw/core.c
@@ -1054,6 +1054,15 @@ static void dwc_caps(struct dma_chan *chan, struct dma_slave_caps *caps)
 	struct dw_dma_chan *dwc = to_dw_dma_chan(chan);
 
 	caps->max_burst = dwc->max_burst;
+
+	/*
+	 * It might be crucial for some devices to have the hardware
+	 * accelerated multi-block transfers supported, aka LLPs in DW DMAC
+	 * notation. So if LLPs are supported then max_sg_nents is set to
+	 * zero which means unlimited number of SG entries can be handled in a
+	 * single DMA transaction, otherwise it's just one SG entry.
+	 */
+	caps->max_sg_nents = dwc->nollp;
 }
 
 int do_dma_probe(struct dw_dma_chip *chip)
-- 
2.26.2