linux-kernel - Re: [REGRESSION] Recent swiotlb DMA_FROM

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Date:   Thu, 24 Mar 2022 06:57:32 +0100
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Halil Pasic <pasic@...ux.ibm.com>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Toke Høiland-Jørgensen <toke@...e.dk>,
        Kalle Valo <kvalo@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Olha Cherevyk <olha.cherevyk@...il.com>,
        iommu <iommu@...ts.linux-foundation.org>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [REGRESSION] Recent swiotlb DMA_FROM_DEVICE fixes break
 ath9k-based AP

On Wed, Mar 23, 2022 at 08:54:08PM +0000, Robin Murphy wrote:
> I'll admit I still never quite grasped the reason for also adding the 
> override to swiotlb_sync_single_for_device() in aa6f8dcbab47, but I think 
> by that point we were increasingly tired and confused and starting to 
> second-guess ourselves (well, I was, at least). I don't think it's wrong 
> per se, but as I said I do think it can bite anyone who's been doing 
> dma_sync_*() wrong but getting away with it until now. If ddbd89deb7d3 
> alone turns out to work OK then I'd be inclined to try a partial revert of 
> just that one hunk.

Agreed.  Let's try that first.

Oleksandr, can you try the patch below:


diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 6db1c475ec827..6c350555e5a1c 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -701,13 +701,10 @@ void swiotlb_tbl_unmap_single(struct device *dev, phys_addr_t tlb_addr,
 void swiotlb_sync_single_for_device(struct device *dev, phys_addr_t tlb_addr,
 		size_t size, enum dma_data_direction dir)
 {
-	/*
-	 * Unconditional bounce is necessary to avoid corruption on
-	 * sync_*_for_cpu or dma_ummap_* when the device didn't overwrite
-	 * the whole lengt of the bounce buffer.
-	 */
-	swiotlb_bounce(dev, tlb_addr, size, DMA_TO_DEVICE);
-	BUG_ON(!valid_dma_direction(dir));
+	if (dir == DMA_TO_DEVICE || dir == DMA_BIDIRECTIONAL)
+		swiotlb_bounce(dev, tlb_addr, size, DMA_TO_DEVICE);
+	else
+		BUG_ON(dir != DMA_FROM_DEVICE);
 }
 
 void swiotlb_sync_single_for_cpu(struct device *dev, phys_addr_t tlb_addr,