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

Nicolas Saenz Julienne
2020-07-27T17:57:06Z

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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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