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From:   Paul Cercueil <paul@crapouillou.net>
To:     Vinod Koul <vkoul@kernel.org>, Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Ralf Baechle <ralf@linux-mips.org>,
        Paul Burton <paul.burton@mips.com>,
        James Hogan <jhogan@kernel.org>,
        Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Cc:     Mathieu Malaterre <malat@debian.org>,
        Daniel Silsby <dansilsby@gmail.com>, dmaengine@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mips@linux-mips.org
Subject: [PATCH 10/14] dmaengine: dma-jz4780: Set DTCn register explicitly
Date:   Tue,  3 Jul 2018 14:32:10 +0200
Message-Id: <20180703123214.23090-11-paul@crapouillou.net>
In-Reply-To: <20180703123214.23090-1-paul@crapouillou.net>
References: <20180703123214.23090-1-paul@crapouillou.net>
Sender: linux-kernel-owner@vger.kernel.org
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From: Daniel Silsby <dansilsby@gmail.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@gmail.com>
---
 drivers/dma/dma-jz4780.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/dma/dma-jz4780.c b/drivers/dma/dma-jz4780.c
index adada2a3a067..64270d53ba57 100644
--- a/drivers/dma/dma-jz4780.c
+++ b/drivers/dma/dma-jz4780.c
@@ -526,6 +526,15 @@ 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));
-- 
2.18.0