linux-kernel - Re: [PATCH 1/1] dma-contiguous: return early for dt case in dma_contiguous

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Date:   Tue, 18 May 2021 19:51:07 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Dong Aisheng <aisheng.dong@....com>,
        iommu@...ts.linux-foundation.org
Cc:     dongas86@...il.com, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 1/1] dma-contiguous: return early for dt case in
 dma_contiguous_reserve

On 2021-05-18 12:28, Dong Aisheng wrote:
> dma_contiguous_reserve() aims to support cmdline case for CMA memory
> reserve. But if users define reserved memory in DT,
> 'dma_contiguous_default_area' will not be 0, then it's meaningless
> to continue to run dma_contiguous_reserve(). So we return early
> if detect 'dma_contiguous_default_area' is unzero.

But dma_contiguous_default_area *shouldn't* be set if the command-line 
argument is present - see the "if (size_cmdline != -1 && default_cma)" 
part of rmem_cma_setup(). Are you seeing something different in practice?

> Cc: Christoph Hellwig <hch@....de>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Robin Murphy <robin.murphy@....com>
> Signed-off-by: Dong Aisheng <aisheng.dong@....com>
> ---
>   kernel/dma/contiguous.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
> index 3d63d91cba5c..ebade9f43eff 100644
> --- a/kernel/dma/contiguous.c
> +++ b/kernel/dma/contiguous.c
> @@ -171,6 +171,9 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
>   	phys_addr_t selected_limit = limit;
>   	bool fixed = false;
>   
> +	if (dma_contiguous_default_area)
> +		return;
> +
>   	pr_debug("%s(limit %08lx)\n", __func__, (unsigned long)limit);
>   
>   	if (size_cmdline != -1) {
> @@ -191,7 +194,7 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
>   #endif
>   	}
>   
> -	if (selected_size && !dma_contiguous_default_area) {
> +	if (selected_size) {

Either way, does skipping a handful of trivial calculations and a 
debugging message really matter even when it is redundant? I can't 
imagine it has any measurable effect on boot times...

Robin.

>   		pr_debug("%s: reserving %ld MiB for global area\n", __func__,
>   			 (unsigned long)selected_size / SZ_1M);
>   
>