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 00:24:44 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Vladimir Murzin <vladimir.murzin@....com>
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

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..)

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
 
 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)
 {