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

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Date:   Mon, 20 Jun 2022 19:20:11 -0500
From:   Samuel Holland <samuel@...lland.org>
To:     Jernej Škrabec <jernej.skrabec@...il.com>,
        Chen-Yu Tsai <wens@...e.org>, Vinod Koul <vkoul@...nel.org>
Cc:     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

Hi Jernej,

On 6/20/22 1:28 PM, Jernej Škrabec wrote:
> 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.

Thanks for checking this. A10 and A20 use a separate driver (sun4i-dma). That
driver will also benefit from setting the max segment size, so I will send a
patch for it.

I think all of the variants supported by sun6i-dma have the same segment size
capability, so no quirk is needed here.

Regards,
Samuel

>>
>> 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
>>
>>
> 
>