linux-kernel - Re: [PATCH 2/2] arm-nommu: Add use_reserved_mem() to check if device support reserved memory

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Date:   Wed, 10 Jun 2020 09:19:45 +0100
From:   Vladimir Murzin <vladimir.murzin@....com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     dillon.minfei@...il.com, robh+dt@...nel.org,
        mcoquelin.stm32@...il.com, alexandre.torgue@...com,
        linux@...linux.org.uk, kstewart@...uxfoundation.org,
        allison@...utok.net, info@...ux.net, tglx@...utronix.de,
        devicetree@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] arm-nommu: Add use_reserved_mem() to check if device
 support reserved memory

On 6/10/20 8:24 AM, Christoph Hellwig wrote:
> Ok, I finally found the original patch from Vladimir.  Comments below:
> 
>> +++ b/kernel/dma/direct.c
>> @@ -456,14 +456,14 @@ int dma_direct_mmap(struct device *dev, struct vm_area_struct *vma,
>>  #else /* CONFIG_MMU */
>>  bool dma_direct_can_mmap(struct device *dev)
>>  {
>> -	return false;
>> +	return true;
>>  }
>>  
>>  int dma_direct_mmap(struct device *dev, struct vm_area_struct *vma,
>>  		void *cpu_addr, dma_addr_t dma_addr, size_t size,
>>  		unsigned long attrs)
>>  {
>> -	return -ENXIO;
>> +	return vm_iomap_memory(vma, vma->vm_start, (vma->vm_end - vma->vm_start));;
> 
> I think we should try to reuse the mmu dma_direct_mmap implementation,
> which does about the same.  This version has been compile tested on
> arm-nommu only, let me know what you think: (btw, a nommu_defconfig of
> some kind for arm would be nice..)

Catch-all nommu_defconfig is not easy for ARM, AFAIK folk carry few hacks
for randconfig...

Meanwhile, known working NOMMU configs

$ git grep "# CONFIG_MMU is not set" arch/arm/configs/
arch/arm/configs/efm32_defconfig:# CONFIG_MMU is not set
arch/arm/configs/lpc18xx_defconfig:# CONFIG_MMU is not set
arch/arm/configs/mps2_defconfig:# CONFIG_MMU is not set
arch/arm/configs/stm32_defconfig:# CONFIG_MMU is not set
arch/arm/configs/vf610m4_defconfig:# CONFIG_MMU is not set

> 
> diff --git a/kernel/dma/Kconfig b/kernel/dma/Kconfig
> index d006668c0027d2..e0dae570a51530 100644
> --- a/kernel/dma/Kconfig
> +++ b/kernel/dma/Kconfig
> @@ -71,6 +71,7 @@ config SWIOTLB
>  # in the pagetables
>  #
>  config DMA_NONCOHERENT_MMAP
> +	default y if !MMU
>  	bool

Nit: def_bool !MMU

>  
>  config DMA_REMAP
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 0a4881e59aa7d6..9ec6a5c3fc578c 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -459,7 +459,6 @@ int dma_direct_get_sgtable(struct device *dev, struct sg_table *sgt,
>  	return ret;
>  }
>  
> -#ifdef CONFIG_MMU
>  bool dma_direct_can_mmap(struct device *dev)
>  {
>  	return dev_is_dma_coherent(dev) ||
> @@ -485,19 +484,6 @@ int dma_direct_mmap(struct device *dev, struct vm_area_struct *vma,
>  	return remap_pfn_range(vma, vma->vm_start, pfn + vma->vm_pgoff,
>  			user_count << PAGE_SHIFT, vma->vm_page_prot);
>  }
> -#else /* CONFIG_MMU */
> -bool dma_direct_can_mmap(struct device *dev)
> -{
> -	return false;
> -}
> -
> -int dma_direct_mmap(struct device *dev, struct vm_area_struct *vma,
> -		void *cpu_addr, dma_addr_t dma_addr, size_t size,
> -		unsigned long attrs)
> -{
> -	return -ENXIO;
> -}
> -#endif /* CONFIG_MMU */
>  
>  int dma_direct_supported(struct device *dev, u64 mask)
>  {
> 

LGTM. FWIW:

Reviewed-by: Vladimir Murzin <vladimir.murzin@....com>