Re: [PATCH v3 10/10] dmaengine: dw: Initialize max_sg_nents with nollp flag
From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Date: 2020-05-28 14:56:55
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dmaengine, linux-devicetree, lkml
On Wed, May 27, 2020 at 01:50:21AM +0300, Serge Semin wrote:
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 hunk ↗ jump to hunk
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. + */
+ 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.
} int do_dma_probe(struct dw_dma_chip *chip) -- 2.26.2
-- With Best Regards, Andy Shevchenko