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Subject: Re: [PATCH wireless-drivers 2/2] mt76: dma: fix buffer unmap with
 non-linear skbs
To:     Lorenzo Bianconi <lorenzo@kernel.org>, kvalo@codeaurora.org
Cc:     linux-wireless@vger.kernel.org, sgruszka@redhat.com,
        lorenzo.bianconi@redhat.com, oleksandr@natalenko.name,
        netdev@vger.kernel.org
References: <cover.1571868221.git.lorenzo@kernel.org>
 <1f7560e10edd517bfd9d3c0dd9820e6f420726b6.1571868221.git.lorenzo@kernel.org>
From:   Felix Fietkau <nbd@nbd.name>
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Date:   Thu, 24 Oct 2019 08:30:22 +0200
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On 2019-10-24 00:23, Lorenzo Bianconi wrote:
> mt76 dma layer is supposed to unmap skb data buffers while keep txwi
> mapped on hw dma ring. At the moment mt76 wrongly unmap txwi or does
> not unmap data fragments in even positions for non-linear skbs. This
> issue may result in hw hangs with A-MSDU if the system relies on IOMMU
> or SWIOTLB. Fix this behaviour properly unmapping data fragments on
> non-linear skbs.
> 
> Fixes: 17f1de56df05 ("mt76: add common code shared between multiple chipsets")
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> ---
>  drivers/net/wireless/mediatek/mt76/dma.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/wireless/mediatek/mt76/dma.c b/drivers/net/wireless/mediatek/mt76/dma.c
> index c747eb24581c..8c27956875e7 100644
> --- a/drivers/net/wireless/mediatek/mt76/dma.c
> +++ b/drivers/net/wireless/mediatek/mt76/dma.c
> @@ -93,11 +93,14 @@ static void
>  mt76_dma_tx_cleanup_idx(struct mt76_dev *dev, struct mt76_queue *q, int idx,
>  			struct mt76_queue_entry *prev_e)
>  {
> -	struct mt76_queue_entry *e = &q->entry[idx];
>  	__le32 __ctrl = READ_ONCE(q->desc[idx].ctrl);
> +	struct mt76_queue_entry *e = &q->entry[idx];
>  	u32 ctrl = le32_to_cpu(__ctrl);
> +	bool mcu = e->skb && !e->txwi;
> +	bool first = e->skb == DMA_DUMMY_DATA || e->txwi == DMA_DUMMY_DATA ||
> +		     (e->skb && !skb_is_nonlinear(e->skb));
It seems to me that these conditions could be true not just for the
first entry, but also following entries except for the last one.
I think we should add a 'bool has_txwi' field in struct mt76_queue_entry
to indicate that the first dma address points to a txwi that should not
be unmapped.

- Felix