linux-kernel - Re: [PATCH] dmaengine: rcar-dmac: enable descriptor mode on 40bit

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Message-ID: <1610601.S7DYfbvybr@avalon>
Date:   Tue, 07 Mar 2017 12:33:06 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>
Cc:     Vinod Koul <vinod.koul@...el.com>,
        Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dmaengine: rcar-dmac: enable descriptor mode on 40bit

Hello Morimoto-san,

Thank you for the patch.

On Tuesday 07 Mar 2017 01:10:32 Kuninori Morimoto wrote:
> From: Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>
> 
> SYS-DMAC can use 40bit address transfer, and it supports Descriptor
> Mode too. Current SYS-DMAC driver disables Descriptor Mode if it was
> 40bit address today. But it can use Descriptor Mode with 40bit if
> transfer Source/Destination address are located in same 4GiB region
> in the 40 bit address space.
> This patch enables it if all condition was clear
> 
> Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>
> ---
>  drivers/dma/sh/rcar-dmac.c | 65 +++++++++++++++++++++++-------------------
>  1 file changed, 36 insertions(+), 29 deletions(-)
> 
> diff --git a/drivers/dma/sh/rcar-dmac.c b/drivers/dma/sh/rcar-dmac.c
> index 48b22d5..2ebd2be 100644
> --- a/drivers/dma/sh/rcar-dmac.c
> +++ b/drivers/dma/sh/rcar-dmac.c

[snip]

> @@ -855,8 +859,12 @@ static void rcar_dmac_chan_configure_desc(struct
> rcar_dmac_chan *chan, unsigned int nchunks = 0;
>  	unsigned int max_chunk_size;
>  	unsigned int full_size = 0;
> -	bool highmem = false;
> +	bool cross_boundary = false;
>  	unsigned int i;
> +#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
> +	u32 high_src_addr = 0;
> +	u32 high_dst_addr = 0;
> +#endif
> 
>  	desc = rcar_dmac_desc_get(chan);
>  	if (!desc)
> @@ -885,25 +893,6 @@ static void rcar_dmac_chan_configure_desc(struct
> rcar_dmac_chan *chan, while (len) {
>  			unsigned int size = min(len, max_chunk_size);
> 
> -#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
> -			/*
> -			 * Prevent individual transfers from crossing 4GB
> -			 * boundaries.
> -			 */
> -			if (dev_addr >> 32 != (dev_addr + size - 1) >> 32)
> -				size = ALIGN(dev_addr, 1ULL << 32) - dev_addr;
> -			if (mem_addr >> 32 != (mem_addr + size - 1) >> 32)
> -				size = ALIGN(mem_addr, 1ULL << 32) - mem_addr;
> -
> -			/*
> -			 * Check if either of the source or destination 
address
> -			 * can't be expressed in 32 bits. If so we can't use
> -			 * hardware descriptor lists.
> -			 */
> -			if (dev_addr >> 32 || mem_addr >> 32)
> -				highmem = true;
> -#endif
> -
>  			chunk = rcar_dmac_xfer_chunk_get(chan);
>  			if (!chunk) {
>  				rcar_dmac_desc_put(chan, desc);
> @@ -918,6 +907,26 @@ static void rcar_dmac_chan_configure_desc(struct
> rcar_dmac_chan *chan, chunk->dst_addr = dev_addr;
>  			}
> 
> +#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
> +			if (i == 0) {
> +				high_src_addr = chunk->src_addr >> 32;
> +				high_dst_addr = chunk->dst_addr >> 32;
> +			}

This will be triggered for every iteration of the while loop, so if one sg 
entry crosses a 32-bit boundary, you won't detect it.

I know it will mean one additional #ifdef, but I think you should move that 
code out of the while loop as I previously proposed.

> +			/*
> +			 * Prevent individual transfers from crossing 4GB
> +			 * boundaries.
> +			 */
> +			if (dev_addr >> 32 != (dev_addr + size - 1) >> 32)
> +				size = ALIGN(dev_addr, 1ULL << 32) - dev_addr;
> +			if (mem_addr >> 32 != (mem_addr + size - 1) >> 32)
> +				size = ALIGN(mem_addr, 1ULL << 32) - mem_addr;

This would then become

			if (dev_addr >> 32 != (dev_addr + size - 1) >> 32) {
				size = ALIGN(dev_addr, 1ULL << 32) - dev_addr;
				cross_boundary = true;
			}
			if (mem_addr >> 32 != (mem_addr + size - 1) >> 32) {
				size = ALIGN(mem_addr, 1ULL << 32) - mem_addr;
				cross_boundary = true;
			}


> +			if ((chunk->src_addr >> 32 != high_src_addr) ||
> +			    (chunk->dst_addr >> 32 != high_dst_addr))
> +				cross_boundary = true;

And this check can also be moved just before the while loop.

> +#endif
> +
>  			chunk->size = size;
> 
>  			dev_dbg(chan->chan.device->dev,
> @@ -943,13 +952,11 @@ static void rcar_dmac_chan_configure_desc(struct
> rcar_dmac_chan *chan, * Use hardware descriptor lists if possible when more
> than one chunk * needs to be transferred (otherwise they don't make much
> sense). *
> -	 * The highmem check currently covers the whole transfer. As an
> -	 * optimization we could use descriptor lists for consecutive lowmem
> -	 * chunks and direct manual mode for highmem chunks. Whether the
> -	 * performance improvement would be significant enough compared to the
> -	 * additional complexity remains to be investigated.
> +	 * Source/Destination address should be located in same 4GiB region
> +	 * in the 40bit address space when it uses Hardware descriptor,
> +	 * and cross_boundary is checking it.
>  	 */
> -	desc->hwdescs.use = !highmem && nchunks > 1;
> +	desc->hwdescs.use = !cross_boundary && nchunks > 1;
>  	if (desc->hwdescs.use) {
>  		if (rcar_dmac_fill_hwdesc(chan, desc) < 0)
>  			desc->hwdescs.use = false;

-- 
Regards,

Laurent Pinchart