linux-kernel - Re: [PATCH] dma-pool: Do not allocate pool memory from CMA

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Date:   Mon, 27 Jul 2020 19:56:56 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Christoph Hellwig <hch@....de>,
        Amit Pundir <amit.pundir@...aro.org>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        David Rientjes <rientjes@...gle.com>,
        linux-rpi-kernel@...ts.infradead.org, jeremy.linton@....com,
        iommu@...ts.linux-foundation.org,
        lkml <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>,
        Sumit Semwal <sumit.semwal@...aro.org>
Subject: Re: [PATCH] dma-pool: Do not allocate pool memory from CMA

Hi Christoph,
thanks for having a look at this!

On Fri, 2020-07-24 at 15:41 +0200, Christoph Hellwig wrote:
> Yes, the iommu is an interesting case, and the current code is
> wrong for that.

Care to expand on this? I do get that checking dma_coherent_ok() on memory
that'll later on be mapped into an iommu is kind of silly, although I think
harmless in Amir's specific case, since devices have wide enough dma-ranges. Is
there more to it?

> Can you try the patch below?  It contains a modified version of Nicolas'
> patch to try CMA again for the expansion and a new (for now hackish) way to
> not apply the addressability check for dma-iommu allocations.
> 
> diff --git a/kernel/dma/pool.c b/kernel/dma/pool.c
> index 6bc74a2d51273e..ec5e525d2b9309 100644
> --- a/kernel/dma/pool.c
> +++ b/kernel/dma/pool.c
> @@ -3,7 +3,9 @@
>   * Copyright (C) 2012 ARM Ltd.
>   * Copyright (C) 2020 Google LLC
>   */
> +#include <linux/cma.h>
>  #include <linux/debugfs.h>
> +#include <linux/dma-contiguous.h>
>  #include <linux/dma-direct.h>
>  #include <linux/dma-noncoherent.h>
>  #include <linux/init.h>
> @@ -55,6 +57,31 @@ static void dma_atomic_pool_size_add(gfp_t gfp, size_t
> size)
>  		pool_size_kernel += size;
>  }
>  
> +static bool cma_in_zone(gfp_t gfp)
> +{
> +	phys_addr_t end;
> +	unsigned long size;
> +	struct cma *cma;
> +
> +	cma = dev_get_cma_area(NULL);
> +	if (!cma)
> +		return false;
> +
> +	size = cma_get_size(cma);
> +	if (!size)
> +		return false;
> +	end = cma_get_base(cma) - memblock_start_of_DRAM() + size - 1;
> +
> +	/* CMA can't cross zone boundaries, see cma_activate_area() */
> +	if (IS_ENABLED(CONFIG_ZONE_DMA) && (gfp & GFP_DMA) &&
> +	    end <= DMA_BIT_MASK(zone_dma_bits))
> +		return true;
> +	if (IS_ENABLED(CONFIG_ZONE_DMA32) && (gfp & GFP_DMA32) &&
> +	    end <= DMA_BIT_MASK(32))
> +		return true;
> +	return true;

IIUC this will always return true given a CMA is present. Which reverts to the
previous behaviour (previous as in breaking some rpi4 setups), isn't it?

Regards,
Nicolas


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