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

[Alexander Lobakin <aleksander.lobakin@intel.com>]

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
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>>
>>
>> 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