linux-kernel - Re: [PATCH 4/5] dma-direct: implement complete bus_dma

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Date:   Thu, 27 Sep 2018 15:58:04 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 4/5] dma-direct: implement complete bus_dma_mask handling

On 20/09/18 19:52, Christoph Hellwig wrote:
> Instead of rejecting devices with a too small bus_dma_mask we can handle
> by taking the bus dma_mask into account for allocations and bounce
> buffering decisions.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>   include/linux/dma-direct.h |  3 ++-
>   kernel/dma/direct.c        | 21 +++++++++++----------
>   2 files changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
> index b79496d8c75b..fbca184ff5a0 100644
> --- a/include/linux/dma-direct.h
> +++ b/include/linux/dma-direct.h
> @@ -27,7 +27,8 @@ static inline bool dma_capable(struct device *dev, dma_addr_t addr, size_t size)
>   	if (!dev->dma_mask)
>   		return false;
>   
> -	return addr + size - 1 <= *dev->dma_mask;
> +	return addr + size - 1 <=
> +		min_not_zero(*dev->dma_mask, dev->bus_dma_mask);
>   }
>   #endif /* !CONFIG_ARCH_HAS_PHYS_TO_DMA */
>   
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 3c404e33d946..64466b7ef67b 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -43,10 +43,11 @@ check_addr(struct device *dev, dma_addr_t dma_addr, size_t size,
>   			return false;
>   		}
>   
> -		if (*dev->dma_mask >= DMA_BIT_MASK(32)) {
> +		if (*dev->dma_mask >= DMA_BIT_MASK(32) || dev->bus_dma_mask) {

Hmm... say *dev->dma_mask is 31 bits and dev->bus_dma_mask is 40 bits 
due to a global DT property, we'll now scream where we didn't before 
even though the bus mask is almost certainly irrelevant - is that desirable?

>   			dev_err(dev,
> -				"%s: overflow %pad+%zu of device mask %llx\n",
> -				caller, &dma_addr, size, *dev->dma_mask);
> +				"%s: overflow %pad+%zu of device mask %llx bus mask %llx\n",
> +				caller, &dma_addr, size,
> +				*dev->dma_mask, dev->bus_dma_mask);
>   		}
>   		return false;
>   	}
> @@ -65,12 +66,18 @@ u64 dma_direct_get_required_mask(struct device *dev)
>   {
>   	u64 max_dma = phys_to_dma_direct(dev, (max_pfn - 1) << PAGE_SHIFT);
>   
> +	if (dev->bus_dma_mask && dev->bus_dma_mask < max_dma)
> +		max_dma = dev->bus_dma_mask;

Again, I think we could just do another min_not_zero() here. A device 
wired to address only one single page of RAM isn't a realistic prospect 
(and we could just flip the -1 and the shift in the max_dma calculation 
if we *really* wanted to support such things).

> +
>   	return (1ULL << (fls64(max_dma) - 1)) * 2 - 1;
>   }
>   
>   static gfp_t __dma_direct_optimal_gfp_mask(struct device *dev, u64 dma_mask,
>   		u64 *phys_mask)
>   {
> +	if (dev->bus_dma_mask && dev->bus_dma_mask < dma_mask)
> +		dma_mask = dev->bus_dma_mask;
> +

Similarly, can't we assume dma_mask to be nonzero here too? It feels 
like we really shouldn't have managed to get this far without one.

Robin.

>   	if (force_dma_unencrypted())
>   		*phys_mask = __dma_to_phys(dev, dma_mask);
>   	else
> @@ -87,7 +94,7 @@ static gfp_t __dma_direct_optimal_gfp_mask(struct device *dev, u64 dma_mask,
>   static bool dma_coherent_ok(struct device *dev, phys_addr_t phys, size_t size)
>   {
>   	return phys_to_dma_direct(dev, phys) + size - 1 <=
> -			dev->coherent_dma_mask;
> +			min_not_zero(dev->coherent_dma_mask, dev->bus_dma_mask);
>   }
>   
>   void *dma_direct_alloc_pages(struct device *dev, size_t size,
> @@ -291,12 +298,6 @@ int dma_direct_supported(struct device *dev, u64 mask)
>   	if (mask < phys_to_dma(dev, DMA_BIT_MASK(32)))
>   		return 0;
>   #endif
> -	/*
> -	 * Upstream PCI/PCIe bridges or SoC interconnects may not carry
> -	 * as many DMA address bits as the device itself supports.
> -	 */
> -	if (dev->bus_dma_mask && mask > dev->bus_dma_mask)
> -		return 0;
>   	return 1;
>   }
>   
>