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

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Date:   Mon, 20 Jun 2022 20:28:46 +0200
From:   Jernej Škrabec <jernej.skrabec@...il.com>
To:     Chen-Yu Tsai <wens@...e.org>, Vinod Koul <vkoul@...nel.org>,
        Samuel Holland <samuel@...lland.org>
Cc:     Samuel Holland <samuel@...lland.org>, dmaengine@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-sunxi@...ts.linux.dev
Subject: Re: [PATCH] dmaengine: sun6i: Set the maximum segment size

Dne petek, 17. junij 2022 ob 05:42:09 CEST je Samuel Holland napisal(a):
> 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.

At least A10 user manual says 128k max in description for Byte Counter 
register (0x100+N*0x20+0xC), although field size is defined as 23:0, but that's 
still less than 2^25-1. A20 supports only 128k too according to manual. New 
quirk should be introduced for this.

Best regards,
Jernej

> 
> Exposing the real segment size limit (instead of the 64k default)
> reduces the number of SG list segments needed for a transaction.
> 
> 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.
> 
>  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..1425f87d97b7 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, DMA_BIT_MASK(25));
> +
>  	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);
> -- 
> 2.35.1
> 
>