Re: [PATCH net v1] net: stmmac: TSO: Fix unbalanced DMA map/unmap for non-paged SKB data

[Jon Hunter <jonathanh@nvidia.com>]

Hi Russell,

On 04/12/2024 16:39, Russell King (Oracle) wrote:
> On Wed, Dec 04, 2024 at 04:58:34PM +0100, Thierry Reding wrote:
>> This doesn't match the location from earlier, but at least there's
>> something afoot here that needs fixing. I suppose this could simply be
>> hiding any subsequent errors, so once this is fixed we might see other
>> similar issues.
> 
> Well, having a quick look at this, the first thing which stands out is:
> 
> In stmmac_tx_clean(), we have:
> 
>                  if (likely(tx_q->tx_skbuff_dma[entry].buf &&
>                             tx_q->tx_skbuff_dma[entry].buf_type != STMMAC_TXBUF_T
> _XDP_TX)) {
>                          if (tx_q->tx_skbuff_dma[entry].map_as_page)
>                                  dma_unmap_page(priv->device,
>                                                 tx_q->tx_skbuff_dma[entry].buf,
>                                                 tx_q->tx_skbuff_dma[entry].len,
>                                                 DMA_TO_DEVICE);
>                          else
>                                  dma_unmap_single(priv->device,
>                                                   tx_q->tx_skbuff_dma[entry].buf,
>                                                   tx_q->tx_skbuff_dma[entry].len,
>                                                   DMA_TO_DEVICE);
>                          tx_q->tx_skbuff_dma[entry].buf = 0;
>                          tx_q->tx_skbuff_dma[entry].len = 0;
>                          tx_q->tx_skbuff_dma[entry].map_as_page = false;
>                  }
> 
> So, tx_skbuff_dma[entry].buf is expected to point appropriately to the
> DMA region.
> 
> Now if we look at stmmac_tso_xmit():
> 
>          des = dma_map_single(priv->device, skb->data, skb_headlen(skb),
>                               DMA_TO_DEVICE);
>          if (dma_mapping_error(priv->device, des))
>                  goto dma_map_err;
> 
>          if (priv->dma_cap.addr64 <= 32) {
> ...
>          } else {
> ...
>                  des += proto_hdr_len;
> ...
> 	}
> 
>          tx_q->tx_skbuff_dma[tx_q->cur_tx].buf = des;
>          tx_q->tx_skbuff_dma[tx_q->cur_tx].len = skb_headlen(skb);
>          tx_q->tx_skbuff_dma[tx_q->cur_tx].map_as_page = false;
>          tx_q->tx_skbuff_dma[tx_q->cur_tx].buf_type = STMMAC_TXBUF_T_SKB;
> 
> This will result in stmmac_tx_clean() calling dma_unmap_single() using
> "des" and "skb_headlen(skb)" as the buffer start and length.
> 
> One of the requirements of the DMA mapping API is that the DMA handle
> returned by the map operation will be passed into the unmap function.
> Not something that was offset. The length will also be the same.
> 
> We can clearly see above that there is a case where the DMA handle has
> been offset by proto_hdr_len, and when this is so, the value that is
> passed into the unmap operation no longer matches this requirement.
> 
> So, a question to the reporter - what is the value of
> priv->dma_cap.addr64 in your failing case? You should see the value
> in the "Using %d/%d bits DMA host/device width" kernel message.

It is ...

  dwc-eth-dwmac 2490000.ethernet: Using 40/40 bits DMA host/device width

Thanks
Jon

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