linux-kernel - Re: [RFC 4/5] dma-direct: Allocate dma pages directly if global pool allocation fails

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Date:   Mon, 26 Jul 2021 09:01:00 +0200
From:   Christoph Hellwig <hch@....de>
To:     Atish Patra <atish.patra@....com>
Cc:     linux-kernel@...r.kernel.org, Albert Ou <aou@...s.berkeley.edu>,
        Christoph Hellwig <hch@....de>, devicetree@...r.kernel.org,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Frank Rowand <frowand.list@...il.com>,
        Guo Ren <guoren@...ux.alibaba.com>,
        iommu@...ts.linux-foundation.org, linux-riscv@...ts.infradead.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Rob Herring <robh+dt@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Tobias Klauser <tklauser@...tanz.ch>
Subject: Re: [RFC 4/5] dma-direct: Allocate dma pages directly if global
 pool allocation fails

On Fri, Jul 23, 2021 at 02:40:30PM -0700, Atish Patra wrote:
> DMA_GLOBAL_POOL config may be enabled for platforms where global pool is
> not supported because a generic defconfig is expected to boot on different
> platforms. Specifically, some RISC-V platforms may use global pool for
> non-coherent devices while some other platforms are completely coherent.
> However, it is expected that single kernel image must boot on all the
> platforms.
> 
> Continue the dma direct allocation if a allocation from global pool failed.
> This indicates that the platform is relying on some other method (direct
> remap) or just have coherent devices.
> 
> Signed-off-by: Atish Patra <atish.patra@....com>
> ---
>  kernel/dma/direct.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index d1d0258ed6d0..984ea776f099 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -161,8 +161,11 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>  		return arch_dma_alloc(dev, size, dma_handle, gfp, attrs);
>  
>  	if (IS_ENABLED(CONFIG_DMA_GLOBAL_POOL) &&
> -	    !dev_is_dma_coherent(dev))
> -		return dma_alloc_from_global_coherent(dev, size, dma_handle);
> +	    !dev_is_dma_coherent(dev)) {
> +		ret = dma_alloc_from_global_coherent(dev, size, dma_handle);
> +		if (ret)
> +			return ret;

This will now silently return normal non-cache coherent memory when
the global pool allocation fails, and thus is completely broken.