linux-kernel - Re: [RFC PATCH] cma: make number of CMA areas dynamic, remove CONFIG_CMA

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Message-ID: <20200916043207.GA713@infradead.org>
Date:   Wed, 16 Sep 2020 05:32:07 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        Roman Gushchin <guro@...com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Joonsoo Kim <js1304@...il.com>,
        Rik van Riel <riel@...riel.com>,
        Aslan Bakirov <aslan@...com>, Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH] cma: make number of CMA areas dynamic, remove
 CONFIG_CMA_AREAS

On Wed, Sep 02, 2020 at 08:02:04PM -0700, Mike Kravetz wrote:
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -383,25 +383,34 @@ postcore_initcall(atomic_pool_init);
>  struct dma_contig_early_reserve {
>  	phys_addr_t base;
>  	unsigned long size;
> +	struct list_head areas;
>  };
>  
> +static __initdata LIST_HEAD(dma_mmu_remap_areas);
>  
>  void __init dma_contiguous_early_fixup(phys_addr_t base, unsigned long size)
>  {
> +	struct dma_contig_early_reserve *d;
> +
> +	d = memblock_alloc(sizeof(struct dma_contig_early_reserve),
> +			sizeof(void *));
> +	if (!d) {
> +		pr_err("Unable to allocate dma_contig_early_reserve struct!\n");
> +		return;
> +	}
> +
> +	d->base = base;
> +	d->size = size;
> +	list_add_tail(&d->areas, &dma_mmu_remap_areas);
>  }

I wonder if struct cma should grow a flags or type field, so that the
arm code can simply use cma_for_each_area to iterate the CMA areas for
the DMA fixup, and we can remove the extra list and the magic hook.

> +/* modify here */
> +LIST_HEAD(cma_areas);

What does this comment mean?

> +static unsigned int cma_area_count;

It seems this is only used to provide a default name for the CMA
areas, but all areas actually provide a name, so I think we can drop
the default naming and the cma_area_count variable entirely.

>  	if (!size || !memblock_is_region_reserved(base, size))
>  		return -EINVAL;
>  
> +
>  	/* ensure minimal alignment required by mm core */

This adds a spurious empty line.

>  static int __init cma_debugfs_init(void)
>  {
>  	struct dentry *cma_debugfs_root;
> -	int i;
> +	struct cma *c;
>  
>  	cma_debugfs_root = debugfs_create_dir("cma", NULL);
>  
> -	for (i = 0; i < cma_area_count; i++)
> -		cma_debugfs_add_one(&cma_areas[i], cma_debugfs_root);
> +	list_for_each_entry(c, &cma_areas, areas)
> +		cma_debugfs_add_one(c, cma_debugfs_root);

I think this should use cma_for_each_area, that way cma_areas can be
keep static in cma.c.