linux-kernel - Re: [PATCH] dma-mapping: Reduce dma_mapping

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Date:   Tue, 01 Aug 2017 09:58:07 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Robin Murphy <robin.murphy@....com>, hch@....de
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-mapping: Reduce dma_mapping_error() inline bloat

Hi Robin,

On 2017-07-24 19:29, Robin Murphy wrote:
> Thanks to the nested inlining, all drivers correctly calling
> dma_mapping_error() after a mapping a page or single buffer generate two
> calls to get_arch_dma_ops() per callsite, which all adds up to a fair
> old chunk of useless code, e.g. ~3KB for an arm64 defconfig plus extras:
>
>     text	   data	    bss	    dec	    hex	filename
> 13051391	1503898	 327768	14883057	 e318f1	vmlinux.o.old
> 13050751	1503898	 327768	14882417	 e31671	vmlinux.o.new
>
> Give the compiler a hand by making it clear we want the same ops.

Reviewed-by: Marek Szyprowski <m.szyprowski@...sung.com>

Similar pattern is used in drivers/xen/swiotlb-xen.c for mmap and
get_sgtable. This could be also fixed, although those are not used so
frequently as dma_mapping_error.

> Signed-off-by: Robin Murphy <robin.murphy@....com>
> ---
>   include/linux/dma-mapping.h | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 843ab866e0f4..239e53d12ee8 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -541,10 +541,11 @@ static inline void dma_free_noncoherent(struct device *dev, size_t size,
>   
>   static inline int dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
>   {
> -	debug_dma_mapping_error(dev, dma_addr);
> +	const struct dma_map_ops *ops = get_dma_ops(dev);
>   
> -	if (get_dma_ops(dev)->mapping_error)
> -		return get_dma_ops(dev)->mapping_error(dev, dma_addr);
> +	debug_dma_mapping_error(dev, dma_addr);
> +	if (ops->mapping_error)
> +		return ops->mapping_error(dev, dma_addr);
>   	return 0;
>   }
>   

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland