linux-kernel - [PATCH v2 12/17] dmaengine: dma-jz4780: Set DTCn register explicitly

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Message-Id: <20180718182023.8182-13-paul@crapouillou.net>
Date:   Wed, 18 Jul 2018 20:20:18 +0200
From:   Paul Cercueil <paul@...pouillou.net>
To:     Vinod Koul <vkoul@...nel.org>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>,
        Zubair Lutfullah Kakakhel <Zubair.Kakakhel@...tec.com>
Cc:     Mathieu Malaterre <malat@...ian.org>,
        Daniel Silsby <dansilsby@...il.com>,
        Paul Cercueil <paul@...pouillou.net>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mips@...ux-mips.org
Subject: [PATCH v2 12/17] dmaengine: dma-jz4780: Set DTCn register explicitly

From: Daniel Silsby <dansilsby@...il.com>

Normally, we wouldn't set the channel transfer count register directly
when using descriptor-driven transfers. However, there is no harm in
doing so, and it allows jz4780_dma_desc_residue() to report the correct
residue of an ongoing transfer, no matter when it is called.

Signed-off-by: Daniel Silsby <dansilsby@...il.com>
Tested-by: Mathieu Malaterre <malat@...ian.org>
---
 drivers/dma/dma-jz4780.c | 9 +++++++++
 1 file changed, 9 insertions(+)

 v2: No change

diff --git a/drivers/dma/dma-jz4780.c b/drivers/dma/dma-jz4780.c
index cc2a86844db4..78849131c81d 100644
--- a/drivers/dma/dma-jz4780.c
+++ b/drivers/dma/dma-jz4780.c
@@ -530,6 +530,15 @@ static void jz4780_dma_begin(struct jz4780_dma_chan *jzchan)
 	jz4780_dma_chn_writel(jzdma, jzchan->id, JZ_DMA_REG_DRT,
 			      jzchan->transfer_type);
 
+	/*
+	 * Set the transfer count. This is redundant for a descriptor-driven
+	 * transfer. However, there can be a delay between the transfer start
+	 * time and when DTCn reg contains the new transfer count. Setting
+	 * it explicitly ensures residue is computed correctly at all times.
+	 */
+	jz4780_dma_chn_writel(jzdma, jzchan->id, JZ_DMA_REG_DTC,
+				jzchan->desc->desc[jzchan->curr_hwdesc].dtc);
+
 	/* Write descriptor address and initiate descriptor fetch. */
 	desc_phys = jzchan->desc->desc_phys +
 		    (jzchan->curr_hwdesc * sizeof(*jzchan->desc->desc));
-- 
2.11.0