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

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Date:   Mon, 28 Mar 2022 11:48:36 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Maxime Bizon <mbizon@...ebox.fr>,
        Toke Høiland-Jørgensen <toke@...e.dk>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        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 Sun, 2022-03-27 at 05:15 +0200, Halil Pasic wrote:

> 
> The key here is "sync_sg API, all the parameters must be the same
> as those passed into the single mapping API", but I have to admit,
> I don't understand the *single* in here.
> 

Hah. So I wasn't imagining things after all.

However, as the rest of the thread arrives, this still means it's all
broken ... :)

> The intended meaning of the
> last sentence is that one can do partial sync by choose 
> dma_hande_sync, size_sync such that dma_handle_mapping <= dma_handle_sync
> < dma_handle_mapping + size_mapping and dma_handle_sync + size_sync <=
> dma_handle_mapping + size_mapping. But the direction has to remain the
> same.

Right.

> BTW, the current documented definition of the direction is about the
> data transfer direction between memory and the device, and how the CPU
> is interacting with the memory is not in scope. A quote form the
> documentation.
> 
> """
> ======================= =============================================
> DMA_NONE                no direction (used for debugging)
> DMA_TO_DEVICE           data is going from the memory to the device
> DMA_FROM_DEVICE         data is coming from the device to the memory
> DMA_BIDIRECTIONAL       direction isn't known
> ======================= =============================================
> """
> (Documentation/core-api/dma-api.rst)
> 
> My feeling is, that re-defining the dma direction is not a good idea. But
> I don't think my opinion has much weight here.

However, this basically means that the direction argument to the flush
APIs are completely useless, and we do have to define something
new/else...

johannes