linux-kernel - [PATCH v2] dmaengine: sun6i: Set the maximum segment size

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Date:   Sun,  1 Jan 2023 13:36:04 -0600
From:   Samuel Holland <samuel@...lland.org>
To:     Vinod Koul <vkoul@...nel.org>
Cc:     Samuel Holland <samuel@...lland.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Chen-Yu Tsai <wens@...e.org>, dmaengine@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-sunxi@...ts.linux.dev
Subject: [PATCH v2] dmaengine: sun6i: Set the maximum segment size

The sun6i DMA engine supports segment sizes up to 2^25-1 bytes. This is
explicitly stated in newer SoC documentation (H6, D1), and it is implied
in older documentation by the 25-bit width of the "bytes left in the
current segment" register field.

Exposing the real segment size limit (instead of the 64k default)
reduces the number of SG list segments needed for a transaction.

Reviewed-by: Jernej Skrabec <jernej.skrabec@...il.com>
Signed-off-by: Samuel Holland <samuel@...lland.org>
---
Tested on A64, verified that the maximum ALSA PCM period increased, and
that audio playback still worked.

Changes in v2:
 - Use SZ_32M instead of DMA_BIT_MASK to make the limit more obvious

 drivers/dma/sun6i-dma.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/dma/sun6i-dma.c b/drivers/dma/sun6i-dma.c
index b7557f437936..30667d251e97 100644
--- a/drivers/dma/sun6i-dma.c
+++ b/drivers/dma/sun6i-dma.c
@@ -9,6 +9,7 @@
 
 #include <linux/clk.h>
 #include <linux/delay.h>
+#include <linux/dma-mapping.h>
 #include <linux/dmaengine.h>
 #include <linux/dmapool.h>
 #include <linux/interrupt.h>
@@ -1334,6 +1335,8 @@ static int sun6i_dma_probe(struct platform_device *pdev)
 	INIT_LIST_HEAD(&sdc->pending);
 	spin_lock_init(&sdc->lock);
 
+	dma_set_max_seg_size(&pdev->dev, SZ_32M - 1);
+
 	dma_cap_set(DMA_PRIVATE, sdc->slave.cap_mask);
 	dma_cap_set(DMA_MEMCPY, sdc->slave.cap_mask);
 	dma_cap_set(DMA_SLAVE, sdc->slave.cap_mask);
-- 
2.37.4