[PATCH net] ethernet: ionic: Fix DMA mapping test in `ionic_xdp_post_frame()`

[PATCH net] ethernet: ionic: Fix DMA mapping test in `ionic_xdp_post_frame()`

[Thomas Fourier]

The `ionic_tx_map_frag()` wrapper function is used which returns 0 or a
valid DMA address.  Testing that pointer with `dma_mapping_error()`could
be eroneous since the error value exptected by `dma_mapping_error()` is
not 0 but `DMA_MAPPING_ERROR` which is often ~0.

Fixes: ac8813c0ab7d ("ionic: convert Rx queue buffers to use page_pool")
Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
---
 drivers/net/ethernet/pensando/ionic/ionic_txrx.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/pensando/ionic/ionic_txrx.c b/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
index 2ac59564ded1..beefdc43013e 100644
--- a/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
+++ b/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
@@ -357,7 +357,7 @@ static int ionic_xdp_post_frame(struct ionic_queue *q, struct xdp_frame *frame,
 			} else {
 				dma_addr = ionic_tx_map_frag(q, frag, 0,
 							     skb_frag_size(frag));
-				if (dma_mapping_error(q->dev, dma_addr)) {
+				if (!dma_addr) {
 					ionic_tx_desc_unmap_bufs(q, desc_info);
 					return -EIO;
 				}
-- 
2.43.0

Re: [PATCH net] ethernet: ionic: Fix DMA mapping test in `ionic_xdp_post_frame()`

[Michal Swiatkowski]

On Tue, Jun 17, 2025 at 11:18:36AM +0200, Thomas Fourier wrote:
> The `ionic_tx_map_frag()` wrapper function is used which returns 0 or a
> valid DMA address.  Testing that pointer with `dma_mapping_error()`could
> be eroneous since the error value exptected by `dma_mapping_error()` is
> not 0 but `DMA_MAPPING_ERROR` which is often ~0.
> 
> Fixes: ac8813c0ab7d ("ionic: convert Rx queue buffers to use page_pool")
> Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
> ---
>  drivers/net/ethernet/pensando/ionic/ionic_txrx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/pensando/ionic/ionic_txrx.c b/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
> index 2ac59564ded1..beefdc43013e 100644
> --- a/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
> +++ b/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
> @@ -357,7 +357,7 @@ static int ionic_xdp_post_frame(struct ionic_queue *q, struct xdp_frame *frame,
>  			} else {
>  				dma_addr = ionic_tx_map_frag(q, frag, 0,
>  							     skb_frag_size(frag));
> -				if (dma_mapping_error(q->dev, dma_addr)) {
> +				if (!dma_addr) {
>  					ionic_tx_desc_unmap_bufs(q, desc_info);
>  					return -EIO;

Right, in ionic_tx_map_skb() it is used correctly (like here now).
Thanks for fixing.

Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>

>  				}
> -- 
> 2.43.0

Re: [PATCH net] ethernet: ionic: Fix DMA mapping test in `ionic_xdp_post_frame()`

[Alexander Lobakin]

From: Thomas Fourier <fourier.thomas@gmail.com>
Date: Tue, 17 Jun 2025 11:18:36 +0200

> The `ionic_tx_map_frag()` wrapper function is used which returns 0 or a
> valid DMA address.  Testing that pointer with `dma_mapping_error()`could
> be eroneous since the error value exptected by `dma_mapping_error()` is
> not 0 but `DMA_MAPPING_ERROR` which is often ~0.

BTW I remember clearly that Chris wrote that dma_addr == 0 is a
perfectly valid DMA address, that's why ~0 was picked for error checking.
So maybe it's better to fix the driver. I realize that in the real
world, it's almost impossible to get dma_addr == 0 from dma_map_*(), but
still.

> 
> Fixes: ac8813c0ab7d ("ionic: convert Rx queue buffers to use page_pool")
> Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>

Thanks,
Olek

Re: [PATCH net] ethernet: ionic: Fix DMA mapping test in `ionic_xdp_post_frame()`

[Brett Creeley]

On 6/17/2025 2:18 AM, Thomas Fourier wrote:
> Caution: This message originated from an External Source. Use proper caution when opening attachments, clicking links, or responding.
> 
> 
> The `ionic_tx_map_frag()` wrapper function is used which returns 0 or a
> valid DMA address.  Testing that pointer with `dma_mapping_error()`could
> be eroneous since the error value exptected by `dma_mapping_error()` is
> not 0 but `DMA_MAPPING_ERROR` which is often ~0.
> 
> Fixes: ac8813c0ab7d ("ionic: convert Rx queue buffers to use page_pool")
> Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
> ---
>   drivers/net/ethernet/pensando/ionic/ionic_txrx.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/pensando/ionic/ionic_txrx.c b/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
> index 2ac59564ded1..beefdc43013e 100644
> --- a/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
> +++ b/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
> @@ -357,7 +357,7 @@ static int ionic_xdp_post_frame(struct ionic_queue *q, struct xdp_frame *frame,
>                          } else {
>                                  dma_addr = ionic_tx_map_frag(q, frag, 0,
>                                                               skb_frag_size(frag));
> -                               if (dma_mapping_error(q->dev, dma_addr)) {
> +                               if (!dma_addr) {

Thanks for the fix.

After looking at this and Olek's comment, I think it makes the most 
sense to return DMA_MAPPING_ERROR from ionic_tx_map_frag() and 
ionic_tx_map_single() instead of 0 on failures.

Then any callers would do the following check:

	if (unlikely(dma_addr == DMA_MAPPING_ERROR))
		/* failure path */

Another option is always returning dma_addr regardless of 
success/failure from the ionic_tx_map* functions, but then I'd be 
inclined to use dma_mapping_error() again in the caller. This approach 
seems wrong to me, especially when CONFIG_DMA_API_DEBUG is enabled.

Thanks,

Brett



>                                          ionic_tx_desc_unmap_bufs(q, desc_info);
>                                          return -EIO;
>                                  }
> --
> 2.43.0
> 

[PATCH net,v2] ethernet: ionic: Fix DMA mapping tests

[Thomas Fourier]

Removing wrappers around `dma_map_XXX()` to prevent collision between
0 as a valid address and 0 as an error code.

Fixes: ac8813c0ab7d ("ionic: convert Rx queue buffers to use page_pool")
Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
---
Another solution is to remove the wrappers altogether so that it doesn't
call multiple times the `dma_mapping_error()`.  This also makes the code 
more similar to other calls of the DMA mapping API (even if wrappers
around the DMA API are quite common, checking the return code of mapping
in the wrapper does not seem to be common).

Thomas

 .../net/ethernet/pensando/ionic/ionic_txrx.c  | 70 ++++++-------------
 1 file changed, 22 insertions(+), 48 deletions(-)

diff --git a/drivers/net/ethernet/pensando/ionic/ionic_txrx.c b/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
index 2ac59564ded1..1905790d0c4d 100644
--- a/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
+++ b/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
@@ -12,12 +12,7 @@
 #include "ionic_lif.h"
 #include "ionic_txrx.h"
 
-static dma_addr_t ionic_tx_map_single(struct ionic_queue *q,
-				      void *data, size_t len);
 
-static dma_addr_t ionic_tx_map_frag(struct ionic_queue *q,
-				    const skb_frag_t *frag,
-				    size_t offset, size_t len);
 
 static void ionic_tx_desc_unmap_bufs(struct ionic_queue *q,
 				     struct ionic_tx_desc_info *desc_info);
@@ -320,9 +315,9 @@ static int ionic_xdp_post_frame(struct ionic_queue *q, struct xdp_frame *frame,
 		dma_sync_single_for_device(q->dev, dma_addr,
 					   len, DMA_TO_DEVICE);
 	} else /* XDP_REDIRECT */ {
-		dma_addr = ionic_tx_map_single(q, frame->data, len);
-		if (!dma_addr)
-			return -EIO;
+		dma_addr = dma_map_single(q->dev, frame->data, len, DMA_TO_DEVICE);
+		if (dma_mapping_error(q->dev, dma_addr))
+			goto dma_err;
 	}
 
 	buf_info->dma_addr = dma_addr;
@@ -355,11 +350,12 @@ static int ionic_xdp_post_frame(struct ionic_queue *q, struct xdp_frame *frame,
 							   skb_frag_size(frag),
 							   DMA_TO_DEVICE);
 			} else {
-				dma_addr = ionic_tx_map_frag(q, frag, 0,
-							     skb_frag_size(frag));
+				dma_addr = skb_frag_dma_map(q->dev, frag, 0,
+							    skb_frag_size(frag),
+							    DMA_TO_DEVICE);
 				if (dma_mapping_error(q->dev, dma_addr)) {
 					ionic_tx_desc_unmap_bufs(q, desc_info);
-					return -EIO;
+					goto dma_err;
 				}
 			}
 			bi->dma_addr = dma_addr;
@@ -388,6 +384,12 @@ static int ionic_xdp_post_frame(struct ionic_queue *q, struct xdp_frame *frame,
 	ionic_txq_post(q, ring_doorbell);
 
 	return 0;
+
+dma_err:
+	net_warn_ratelimited("%s: DMA map failed on %s!\n",
+			     dev_name(q->dev), q->name);
+	q_to_tx_stats(q)->dma_map_err++;
+	return -EIO;
 }
 
 int ionic_xdp_xmit(struct net_device *netdev, int n,
@@ -1072,38 +1074,6 @@ int ionic_txrx_napi(struct napi_struct *napi, int budget)
 	return rx_work_done;
 }
 
-static dma_addr_t ionic_tx_map_single(struct ionic_queue *q,
-				      void *data, size_t len)
-{
-	struct device *dev = q->dev;
-	dma_addr_t dma_addr;
-
-	dma_addr = dma_map_single(dev, data, len, DMA_TO_DEVICE);
-	if (unlikely(dma_mapping_error(dev, dma_addr))) {
-		net_warn_ratelimited("%s: DMA single map failed on %s!\n",
-				     dev_name(dev), q->name);
-		q_to_tx_stats(q)->dma_map_err++;
-		return 0;
-	}
-	return dma_addr;
-}
-
-static dma_addr_t ionic_tx_map_frag(struct ionic_queue *q,
-				    const skb_frag_t *frag,
-				    size_t offset, size_t len)
-{
-	struct device *dev = q->dev;
-	dma_addr_t dma_addr;
-
-	dma_addr = skb_frag_dma_map(dev, frag, offset, len, DMA_TO_DEVICE);
-	if (unlikely(dma_mapping_error(dev, dma_addr))) {
-		net_warn_ratelimited("%s: DMA frag map failed on %s!\n",
-				     dev_name(dev), q->name);
-		q_to_tx_stats(q)->dma_map_err++;
-		return 0;
-	}
-	return dma_addr;
-}
 
 static int ionic_tx_map_skb(struct ionic_queue *q, struct sk_buff *skb,
 			    struct ionic_tx_desc_info *desc_info)
@@ -1115,9 +1085,9 @@ static int ionic_tx_map_skb(struct ionic_queue *q, struct sk_buff *skb,
 	skb_frag_t *frag;
 	int frag_idx;
 
-	dma_addr = ionic_tx_map_single(q, skb->data, skb_headlen(skb));
-	if (!dma_addr)
-		return -EIO;
+	dma_addr = dma_map_single(q->dev, skb->data, skb_headlen(skb), DMA_TO_DEVICE);
+	if (dma_mapping_error(q->dev, dma_addr))
+		goto dma_early_fail;
 	buf_info->dma_addr = dma_addr;
 	buf_info->len = skb_headlen(skb);
 	buf_info++;
@@ -1125,8 +1095,8 @@ static int ionic_tx_map_skb(struct ionic_queue *q, struct sk_buff *skb,
 	frag = skb_shinfo(skb)->frags;
 	nfrags = skb_shinfo(skb)->nr_frags;
 	for (frag_idx = 0; frag_idx < nfrags; frag_idx++, frag++) {
-		dma_addr = ionic_tx_map_frag(q, frag, 0, skb_frag_size(frag));
-		if (!dma_addr)
+		dma_addr = skb_frag_dma_map(q->dev, frag, 0, skb_frag_size(frag), DMA_TO_DEVICE);
+		if (dma_mapping_error(q->dev, dma_addr))
 			goto dma_fail;
 		buf_info->dma_addr = dma_addr;
 		buf_info->len = skb_frag_size(frag);
@@ -1147,6 +1117,10 @@ static int ionic_tx_map_skb(struct ionic_queue *q, struct sk_buff *skb,
 	}
 	dma_unmap_single(dev, desc_info->bufs[0].dma_addr,
 			 desc_info->bufs[0].len, DMA_TO_DEVICE);
+dma_early_fail:
+	net_warn_ratelimited("%s: DMA map failed on %s!\n",
+			     dev_name(dev), q->name);
+	q_to_tx_stats(q)->dma_map_err++;
 	return -EIO;
 }
 
-- 
2.43.0

Re: [PATCH net,v2] ethernet: ionic: Fix DMA mapping tests

[Brett Creeley]

On 6/17/2025 11:57 AM, Thomas Fourier wrote:
> Caution: This message originated from an External Source. Use proper caution when opening attachments, clicking links, or responding.
> 
> 
> Removing wrappers around `dma_map_XXX()` to prevent collision between
> 0 as a valid address and 0 as an error code.
> 
> Fixes: ac8813c0ab7d ("ionic: convert Rx queue buffers to use page_pool")
> Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
> ---
> Another solution is to remove the wrappers altogether so that it doesn't
> call multiple times the `dma_mapping_error()`.  This also makes the code
> more similar to other calls of the DMA mapping API (even if wrappers
> around the DMA API are quite common, checking the return code of mapping
> in the wrapper does not seem to be common).

This is one solution, but this requires adding extra goto labels and 
pulling the print(s) outside of the helper functions. It also increases 
the size of the diff from a fixes patch that could be a few lines.

I would prefer returning DMA_MAPPING_ERROR from the 
ionic_tx_map_single() and ionic_tx_map_frag() on failure and checking 
for that in the callers.

Thanks,

Brett

> 
> Thomas
> 
>   .../net/ethernet/pensando/ionic/ionic_txrx.c  | 70 ++++++-------------
>   1 file changed, 22 insertions(+), 48 deletions(-)
> 
> diff --git a/drivers/net/ethernet/pensando/ionic/ionic_txrx.c b/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
> index 2ac59564ded1..1905790d0c4d 100644
> --- a/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
> +++ b/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
> @@ -12,12 +12,7 @@
>   #include "ionic_lif.h"
>   #include "ionic_txrx.h"
> 
> -static dma_addr_t ionic_tx_map_single(struct ionic_queue *q,
> -                                     void *data, size_t len);
> 
> -static dma_addr_t ionic_tx_map_frag(struct ionic_queue *q,
> -                                   const skb_frag_t *frag,
> -                                   size_t offset, size_t len);
> 
>   static void ionic_tx_desc_unmap_bufs(struct ionic_queue *q,
>                                       struct ionic_tx_desc_info *desc_info);
> @@ -320,9 +315,9 @@ static int ionic_xdp_post_frame(struct ionic_queue *q, struct xdp_frame *frame,
>                  dma_sync_single_for_device(q->dev, dma_addr,
>                                             len, DMA_TO_DEVICE);
>          } else /* XDP_REDIRECT */ {
> -               dma_addr = ionic_tx_map_single(q, frame->data, len);
> -               if (!dma_addr)
> -                       return -EIO;
> +               dma_addr = dma_map_single(q->dev, frame->data, len, DMA_TO_DEVICE);
> +               if (dma_mapping_error(q->dev, dma_addr))
> +                       goto dma_err;
>          }
> 
>          buf_info->dma_addr = dma_addr;
> @@ -355,11 +350,12 @@ static int ionic_xdp_post_frame(struct ionic_queue *q, struct xdp_frame *frame,
>                                                             skb_frag_size(frag),
>                                                             DMA_TO_DEVICE);
>                          } else {
> -                               dma_addr = ionic_tx_map_frag(q, frag, 0,
> -                                                            skb_frag_size(frag));
> +                               dma_addr = skb_frag_dma_map(q->dev, frag, 0,
> +                                                           skb_frag_size(frag),
> +                                                           DMA_TO_DEVICE);
>                                  if (dma_mapping_error(q->dev, dma_addr)) {
>                                          ionic_tx_desc_unmap_bufs(q, desc_info);
> -                                       return -EIO;
> +                                       goto dma_err;
>                                  }
>                          }
>                          bi->dma_addr = dma_addr;
> @@ -388,6 +384,12 @@ static int ionic_xdp_post_frame(struct ionic_queue *q, struct xdp_frame *frame,
>          ionic_txq_post(q, ring_doorbell);
> 
>          return 0;
> +
> +dma_err:
> +       net_warn_ratelimited("%s: DMA map failed on %s!\n",
> +                            dev_name(q->dev), q->name);
> +       q_to_tx_stats(q)->dma_map_err++;
> +       return -EIO;
>   }
> 
>   int ionic_xdp_xmit(struct net_device *netdev, int n,
> @@ -1072,38 +1074,6 @@ int ionic_txrx_napi(struct napi_struct *napi, int budget)
>          return rx_work_done;
>   }
> 
> -static dma_addr_t ionic_tx_map_single(struct ionic_queue *q,
> -                                     void *data, size_t len)
> -{
> -       struct device *dev = q->dev;
> -       dma_addr_t dma_addr;
> -
> -       dma_addr = dma_map_single(dev, data, len, DMA_TO_DEVICE);
> -       if (unlikely(dma_mapping_error(dev, dma_addr))) {
> -               net_warn_ratelimited("%s: DMA single map failed on %s!\n",
> -                                    dev_name(dev), q->name);
> -               q_to_tx_stats(q)->dma_map_err++;
> -               return 0;
> -       }
> -       return dma_addr;
> -}
> -
> -static dma_addr_t ionic_tx_map_frag(struct ionic_queue *q,
> -                                   const skb_frag_t *frag,
> -                                   size_t offset, size_t len)
> -{
> -       struct device *dev = q->dev;
> -       dma_addr_t dma_addr;
> -
> -       dma_addr = skb_frag_dma_map(dev, frag, offset, len, DMA_TO_DEVICE);
> -       if (unlikely(dma_mapping_error(dev, dma_addr))) {
> -               net_warn_ratelimited("%s: DMA frag map failed on %s!\n",
> -                                    dev_name(dev), q->name);
> -               q_to_tx_stats(q)->dma_map_err++;
> -               return 0;
> -       }
> -       return dma_addr;
> -}
> 
>   static int ionic_tx_map_skb(struct ionic_queue *q, struct sk_buff *skb,
>                              struct ionic_tx_desc_info *desc_info)
> @@ -1115,9 +1085,9 @@ static int ionic_tx_map_skb(struct ionic_queue *q, struct sk_buff *skb,
>          skb_frag_t *frag;
>          int frag_idx;
> 
> -       dma_addr = ionic_tx_map_single(q, skb->data, skb_headlen(skb));
> -       if (!dma_addr)
> -               return -EIO;
> +       dma_addr = dma_map_single(q->dev, skb->data, skb_headlen(skb), DMA_TO_DEVICE);
> +       if (dma_mapping_error(q->dev, dma_addr))
> +               goto dma_early_fail;
>          buf_info->dma_addr = dma_addr;
>          buf_info->len = skb_headlen(skb);
>          buf_info++;
> @@ -1125,8 +1095,8 @@ static int ionic_tx_map_skb(struct ionic_queue *q, struct sk_buff *skb,
>          frag = skb_shinfo(skb)->frags;
>          nfrags = skb_shinfo(skb)->nr_frags;
>          for (frag_idx = 0; frag_idx < nfrags; frag_idx++, frag++) {
> -               dma_addr = ionic_tx_map_frag(q, frag, 0, skb_frag_size(frag));
> -               if (!dma_addr)
> +               dma_addr = skb_frag_dma_map(q->dev, frag, 0, skb_frag_size(frag), DMA_TO_DEVICE);
> +               if (dma_mapping_error(q->dev, dma_addr))
>                          goto dma_fail;
>                  buf_info->dma_addr = dma_addr;
>                  buf_info->len = skb_frag_size(frag);
> @@ -1147,6 +1117,10 @@ static int ionic_tx_map_skb(struct ionic_queue *q, struct sk_buff *skb,
>          }
>          dma_unmap_single(dev, desc_info->bufs[0].dma_addr,
>                           desc_info->bufs[0].len, DMA_TO_DEVICE);
> +dma_early_fail:
> +       net_warn_ratelimited("%s: DMA map failed on %s!\n",
> +                            dev_name(dev), q->name);
> +       q_to_tx_stats(q)->dma_map_err++;
>          return -EIO;
>   }
> 
> --
> 2.43.0
> 

Re: [PATCH net] ethernet: ionic: Fix DMA mapping test in `ionic_xdp_post_frame()`

[Alexander Lobakin]

From: Brett Creeley <bcreeley@amd.com>
Date: Tue, 17 Jun 2025 09:56:55 -0700

> On 6/17/2025 2:18 AM, Thomas Fourier wrote:
>> Caution: This message originated from an External Source. Use proper
>> caution when opening attachments, clicking links, or responding.
>>
>>
>> The `ionic_tx_map_frag()` wrapper function is used which returns 0 or a
>> valid DMA address.  Testing that pointer with `dma_mapping_error()`could
>> be eroneous since the error value exptected by `dma_mapping_error()` is
>> not 0 but `DMA_MAPPING_ERROR` which is often ~0.
>>
>> Fixes: ac8813c0ab7d ("ionic: convert Rx queue buffers to use page_pool")
>> Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
>> ---
>>   drivers/net/ethernet/pensando/ionic/ionic_txrx.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/ethernet/pensando/ionic/ionic_txrx.c b/
>> drivers/net/ethernet/pensando/ionic/ionic_txrx.c
>> index 2ac59564ded1..beefdc43013e 100644
>> --- a/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
>> +++ b/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
>> @@ -357,7 +357,7 @@ static int ionic_xdp_post_frame(struct ionic_queue
>> *q, struct xdp_frame *frame,
>>                          } else {
>>                                  dma_addr = ionic_tx_map_frag(q, frag, 0,
>>                                                              
>> skb_frag_size(frag));
>> -                               if (dma_mapping_error(q->dev,
>> dma_addr)) {
>> +                               if (!dma_addr) {
> 
> Thanks for the fix.
> 
> After looking at this and Olek's comment, I think it makes the most
> sense to return DMA_MAPPING_ERROR from ionic_tx_map_frag() and
> ionic_tx_map_single() instead of 0 on failures.
> 
> Then any callers would do the following check:
> 
>     if (unlikely(dma_addr == DMA_MAPPING_ERROR))
>         /* failure path */
> 
> Another option is always returning dma_addr regardless of success/
> failure from the ionic_tx_map* functions, but then I'd be inclined to
> use dma_mapping_error() again in the caller. This approach seems wrong
> to me, especially when CONFIG_DMA_API_DEBUG is enabled.

Yup, I'd agree that dma_mapping_error() should be used only for checking
return values of the generic DMA API functions and only once per one
mapping. Otherwise, it's fine to pass DMA_MAPPING_ERROR and test against
it directly (after dma_mapping_error() was called already).

> 
> Thanks,
> 
> Brett

Thanks,
Olek