netdev - Re: [PATCH 07/12] dma-direct: simplify the use atomic pool logic in dma_direct

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Date: Mon, 16 Oct 2023 12:58:47 +0100
From: Robin Murphy <robin.murphy@....com>
To: Christoph Hellwig <hch@....de>, Greg Ungerer <gerg@...ux-m68k.org>,
 iommu@...ts.linux.dev
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Conor Dooley <conor@...nel.org>,
 Geert Uytterhoeven <geert+renesas@...der.be>,
 Magnus Damm <magnus.damm@...il.com>,
 Marek Szyprowski <m.szyprowski@...sung.com>,
 Geert Uytterhoeven <geert@...ux-m68k.org>, Wei Fang <wei.fang@....com>,
 Shenwei Wang <shenwei.wang@....com>, Clark Wang <xiaoning.wang@....com>,
 NXP Linux Team <linux-imx@....com>, linux-m68k@...ts.linux-m68k.org,
 netdev@...r.kernel.org, linux-riscv@...ts.infradead.org,
 linux-renesas-soc@...r.kernel.org, Jim Quinlan <james.quinlan@...adcom.com>
Subject: Re: [PATCH 07/12] dma-direct: simplify the use atomic pool logic in
 dma_direct_alloc

On 16/10/2023 6:47 am, Christoph Hellwig wrote:
> The logic in dma_direct_alloc when to use the atomic pool vs remapping
> grew a bit unreadable.  Consolidate it into a single check, and clean
> up the set_uncached vs remap logic a bit as well.

Reviewed-by: Robin Murphy <robin.murphy@....com>

> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>   kernel/dma/direct.c | 25 ++++++++++---------------
>   1 file changed, 10 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index ec410af1d8a14e..1327d04fa32a25 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -234,27 +234,22 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   					dma_handle);
>   
>   		/*
> -		 * Otherwise remap if the architecture is asking for it.  But
> -		 * given that remapping memory is a blocking operation we'll
> -		 * instead have to dip into the atomic pools.
> +		 * Otherwise we require the architecture to either be able to
> +		 * mark arbitrary parts of the kernel direct mapping uncached,
> +		 * or remapped it uncached.
>   		 */
> +		set_uncached = IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED);
>   		remap = IS_ENABLED(CONFIG_DMA_DIRECT_REMAP);
> -		if (remap) {
> -			if (dma_direct_use_pool(dev, gfp))
> -				return dma_direct_alloc_from_pool(dev, size,
> -						dma_handle, gfp);
> -		} else {
> -			if (!IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED))
> -				return NULL;
> -			set_uncached = true;
> -		}
> +		if (!set_uncached && !remap)
> +			return NULL;
>   	}
>   
>   	/*
> -	 * Decrypting memory may block, so allocate the memory from the atomic
> -	 * pools if we can't block.
> +	 * Remapping or decrypting memory may block, allocate the memory from
> +	 * the atomic pools instead if we aren't allowed block.
>   	 */
> -	if (force_dma_unencrypted(dev) && dma_direct_use_pool(dev, gfp))
> +	if ((remap || force_dma_unencrypted(dev)) &&
> +	    dma_direct_use_pool(dev, gfp))
>   		return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
>   
>   	/* we always manually zero the memory once we are done */