On Thu, May 28, 2020 at 05:56:30PM +0300, Andy Shevchenko wrote:

On Wed, May 27, 2020 at 01:50:21AM +0300, Serge Semin wrote:

quoted

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.

In principal I agree, one nit below.
If you are okay with it, feel free to add my Rb tag.

quoted

Signed-off-by: Serge Semin <redacted>
Cc: Alexey Malahov <redacted>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: devicetree@vger.kernel.org

---

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.
+	 */

quoted

+	caps->max_sg_nents = dwc->nollp;

To be on the safer side I would explicitly do it like

	if (dwc->nollp)
	 /* your nice comment */
	 = 1;
	else
	 /* Unlimited */
	 = 0;

type or content of nollp theoretically can be changed and this will affect maximum segments.

Agree. Though I don't like formatting you suggested. If I add my nice comment
between if-statement and assignment the the former will be look detached from
the if-statement, which seems a bit ugly. So I'd leave the comment above the
whole if-else statement, especially seeing I've already mentioned there about
the unlimited number of SG entries there.

	/*
	 * 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.
	 */
 	if (dwc->nollp)
 		caps->max_sg_nents = 1;
 	else
 		caps->max_sg_nents = 0;

-Sergey

quoted

 }
 
 int do_dma_probe(struct dw_dma_chip *chip)
-- 
2.26.2

-- 
With Best Regards,
Andy Shevchenko