linux-kernel - Re: [PATCH 1/3] dma-pool: factor out a calculate_pool

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Date:   Wed, 17 Aug 2022 13:32:42 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>, iommu@...ts.linux.dev,
        Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] dma-pool: factor out a calculate_pool_size helper

On 2022-08-17 07:06, Christoph Hellwig wrote:
> Add a helper to calculate the pool size from dma_atomic_pool_init,
> and fix up the last max_t to use the proper type.

Hmm, both atomic_pool_size and the argument to __dma_atomic_pool_init() 
where this gets directly passed later are size_t, not to mention that 
the function name says we're calculating a size, so I'd say size_t *is* 
the proper type to return here.

> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>   kernel/dma/pool.c | 17 ++++++++++++-----
>   1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/dma/pool.c b/kernel/dma/pool.c
> index 4d40dcce7604b..56f96678934bf 100644
> --- a/kernel/dma/pool.c
> +++ b/kernel/dma/pool.c
> @@ -184,6 +184,15 @@ static __init struct gen_pool *__dma_atomic_pool_init(size_t pool_size,
>   	return pool;
>   }
>   
> +static unsigned long calculate_pool_size(unsigned long zone_pages)
> +{
> +	unsigned long nr_pages = min_t(unsigned long,
> +				       zone_pages / (SZ_1G / SZ_128K),
> +				       MAX_ORDER_NR_PAGES);

Nit: this is arguably less readable, and objectively one line longer, 
than the original two statements.

Other that those nits though,

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

> +
> +	return max_t(unsigned long, nr_pages << PAGE_SHIFT, SZ_128K);
> +}
> +
>   static int __init dma_atomic_pool_init(void)
>   {
>   	int ret = 0;
> @@ -192,11 +201,9 @@ static int __init dma_atomic_pool_init(void)
>   	 * If coherent_pool was not used on the command line, default the pool
>   	 * sizes to 128KB per 1GB of memory, min 128KB, max MAX_ORDER-1.
>   	 */
> -	if (!atomic_pool_size) {
> -		unsigned long pages = totalram_pages() / (SZ_1G / SZ_128K);
> -		pages = min_t(unsigned long, pages, MAX_ORDER_NR_PAGES);
> -		atomic_pool_size = max_t(size_t, pages << PAGE_SHIFT, SZ_128K);
> -	}
> +	if (!atomic_pool_size)
> +		atomic_pool_size = calculate_pool_size(totalram_pages());
> +
>   	INIT_WORK(&atomic_pool_work, atomic_pool_work_fn);
>   
>   	atomic_pool_kernel = __dma_atomic_pool_init(atomic_pool_size,