linux-kernel - Re: [PATCH 05/18] ARM/dma-mapping: Switch to iommu_dma

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Date:   Mon, 28 Sep 2020 13:32:44 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Robin Murphy <robin.murphy@....com>, hch@....de, joro@...tes.org,
        linux@...linux.org.uk
Cc:     will@...nel.org, inki.dae@...sung.com, sw0312.kim@...sung.com,
        kyungmin.park@...sung.com, agross@...nel.org,
        bjorn.andersson@...aro.org, thierry.reding@...il.com,
        jonathanh@...dia.com, vdumpa@...dia.com, digetx@...il.com,
        matthias.bgg@...il.com, yong.wu@...iatek.com,
        geert+renesas@...der.be, magnus.damm@...il.com, t-kristo@...com,
        s-anna@...com, laurent.pinchart@...asonboard.com,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org,
        linux-samsung-soc@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/18] ARM/dma-mapping: Switch to iommu_dma_ops

Hi Robin,

On 20.08.2020 17:08, Robin Murphy wrote:
> With the IOMMU ops now looking much the same shape as iommu_dma_ops,
> switch them out in favour of the iommu-dma library, currently enhanced
> with temporary workarounds that allow it to also sit underneath the
> arch-specific API. With that in place, we can now start converting the
> remaining IOMMU drivers and consumers to work with IOMMU API default
> domains instead.
>
> Signed-off-by: Robin Murphy <robin.murphy@....com>

I've played a bit longer with this and found that reading the kernel 
virtual address of the buffers allocated via dma_alloc_attrs() from 
dma-iommu ops gives trashes from time to time. It took me a while to 
debug this...

Your conversion misses adding arch_dma_prep_coherent() to arch/arm, so 
the buffers are cleared by the mm allocator, but the caches are NOT 
flushed for the newly allocated buffers.

This fixes the issue:

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index fec3e59215b8..8b60bcc5b14f 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -2,6 +2,7 @@
  config ARM
         bool
         default y
+       select ARCH_HAS_DMA_PREP_COHERENT
         select ARCH_32BIT_OFF_T
         select ARCH_HAS_BINFMT_FLAT
         select ARCH_HAS_DEBUG_VIRTUAL if MMU
diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
index ff6c4962161a..6954681b73da 100644
--- a/arch/arm/mm/dma-mapping.c
+++ b/arch/arm/mm/dma-mapping.c
@@ -266,6 +266,20 @@ static void __dma_clear_buffer(struct page *page, 
size_t size, int coherent_flag
         }
  }

+void arch_dma_prep_coherent(struct page *page, size_t size)
+{
+
+       if (PageHighMem(page)) {
+               phys_addr_t base = __pfn_to_phys(page_to_pfn(page));
+               phys_addr_t end = base + size;
+               outer_flush_range(base, end);
+       } else {
+               void *ptr = page_address(page);
+               dmac_flush_range(ptr, ptr + size);
+               outer_flush_range(__pa(ptr), __pa(ptr) + size);
+       }
+}
+
  /*
   * Allocate a DMA buffer for 'dev' of size 'size' using the
   * specified gfp mask.  Note that 'size' must be page aligned.

I also wonder if it would be better to use per-arch __dma_clear_buffer() 
instead of setting __GFP_ZERO unconditionally in dma-iommu.c. This 
should be faster on ARM with highmem...

 > ...

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland