RE: [PATCH bpf-next v8 4/5] igc: Refactor empty packet insertion into a reusable function

[Song, Yoong Siang]

On Wednesday, February 5, 2025 8:31 PM, Fijalkowski, Maciej <maciej.fijalkowski@xxxxxxxxx> wrote:
>On Wed, Feb 05, 2025 at 10:41:15AM +0800, Song Yoong Siang wrote:
>> Refactor the code for inserting an empty packet into a new function
>> igc_insert_empty_packet(). This change extracts the logic for inserting
>> an empty packet from igc_xmit_frame_ring() into a separate function,
>> allowing it to be reused in future implementations, such as the XDP
>> zero copy transmit function.
>>
>> This patch introduces no functional changes.
>>
>> Signed-off-by: Song Yoong Siang <yoong.siang.song@xxxxxxxxx>
>
>Your SoB should be last in the set of tags.
>

Noted. Thanks for the tips.

>> Reviewed-by: Faizal Rahim <faizal.abdul.rahim@xxxxxxxxxxxxxxx>
>> ---
>>  drivers/net/ethernet/intel/igc/igc_main.c | 42 ++++++++++++-----------
>>  1 file changed, 22 insertions(+), 20 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/intel/igc/igc_main.c
>b/drivers/net/ethernet/intel/igc/igc_main.c
>> index 21f318f12a8d..553d6d82af0d 100644
>> --- a/drivers/net/ethernet/intel/igc/igc_main.c
>> +++ b/drivers/net/ethernet/intel/igc/igc_main.c
>> @@ -1566,6 +1566,26 @@ static bool igc_request_tx_tstamp(struct igc_adapter
>*adapter, struct sk_buff *s
>>  	return false;
>>  }
>>
>> +static void igc_insert_empty_packet(struct igc_ring *tx_ring)
>> +{
>> +	struct igc_tx_buffer *empty_info;
>> +	struct sk_buff *empty;
>> +	void *data;
>> +
>> +	empty_info = &tx_ring->tx_buffer_info[tx_ring->next_to_use];
>> +	empty = alloc_skb(IGC_EMPTY_FRAME_SIZE, GFP_ATOMIC);
>> +	if (!empty)
>> +		return;
>> +
>> +	data = skb_put(empty, IGC_EMPTY_FRAME_SIZE);
>> +	memset(data, 0, IGC_EMPTY_FRAME_SIZE);
>> +
>> +	igc_tx_ctxtdesc(tx_ring, 0, false, 0, 0, 0);
>> +
>> +	if (igc_init_tx_empty_descriptor(tx_ring, empty, empty_info) < 0)
>> +		dev_kfree_skb_any(empty);
>
>I still don't like the fact igc_insert_empty_packet() doesn't communicate
>to caller whether it successfully produced descriptors or not.
>
>Look at this from igc_xmit_frame_ring() POV:
>- at the beginning you peek at Tx ring whether there is required amount of
>  descriptors free to be used
>- but then here's your additional routine which might consume two more
>  descs and you are not aware of the status
>- then you continue to further produce descriptors assuming there is
>  enough space in Tx ring
>
>Right now igc_init_tx_empty_descriptor() returns -EBUSY when ring is full.
>How can that happen in the first place + what if it would *really* happen
>though? You just continue with your Tx flow.
>
>What I'm trying to say here is, at least from correctness POV, you should
>take into the account two potential descriptors for launchtime feature
>when calling igc_maybe_stop_tx(). And igc_init_tx_empty_descriptor()
>should not really care about space in ring, it should be a caller's job to
>call it only when it will be sure it's safe to do so.
>

Agree with you.

In db0b124f02ba ("igc: Enhance Qbv scheduling by using first flag bit"),
the 2 descriptors needed by empty packet is already taken into
consideration by changing igc_maybe_stop_tx(tx_ring, count + 3) to
igc_maybe_stop_tx(tx_ring, count + 5), so not enough ring space issue will not
happened. However, the comment session is not updated, maybe i can update
it in next version of this patch as below:

@@ -1586,6 +1608,7 @@ static netdev_tx_t igc_xmit_frame_ring(struct sk_buff *skb,
         *      + 1 desc for skb_headlen/IGC_MAX_DATA_PER_TXD,
         *      + 2 desc gap to keep tail from touching head,
         *      + 1 desc for context descriptor,
+        *      + 2 desc for inserting an empty packet for launch time,
         * otherwise try next time
         */
        for (f = 0; f < skb_shinfo(skb)->nr_frags; f++)

Since number of descriptor needed is guaranteed. I will take out the
igc_desc_unused() checking in igc_init_tx_empty_descriptor().
However, empty packet insertion might fail due to skb allocation failure
and DMA mapping error. How about i make sure skb allocation and DMA
mapping working, before proceed to fill in igc_tx_buffer, context desc,
and data desc?
IMHO, because these two errors are unlikely to happen, print a kernel
warning msg should be enough.

@@ -1108,20 +1108,12 @@ static int igc_init_empty_frame(struct igc_ring *ring,
        return 0;
 }

-static int igc_init_tx_empty_descriptor(struct igc_ring *ring,
+static void igc_init_tx_empty_descriptor(struct igc_ring *ring,
                                        struct sk_buff *skb,
                                        struct igc_tx_buffer *first)
 {
        union igc_adv_tx_desc *desc;
        u32 cmd_type, olinfo_status;
-       int err;
-
-       if (!igc_desc_unused(ring))
-               return -EBUSY;
-
-       err = igc_init_empty_frame(ring, first, skb);
-       if (err)
-               return err;

        cmd_type = IGC_ADVTXD_DTYP_DATA | IGC_ADVTXD_DCMD_DEXT |
                   IGC_ADVTXD_DCMD_IFCS | IGC_TXD_DCMD |
@@ -1140,8 +1132,6 @@ static int igc_init_tx_empty_descriptor(struct igc_ring *ring,
        ring->next_to_use++;
        if (ring->next_to_use == ring->count)
                ring->next_to_use = 0;
-
-       return 0;
 }

 #define IGC_EMPTY_FRAME_SIZE 60
@@ -1567,6 +1557,38 @@ static bool igc_request_tx_tstamp(struct igc_adapter *adapter, struct sk_buff *s
        return false;
 }

+static void igc_insert_empty_packet(struct igc_ring *tx_ring)
+{
+       struct igc_tx_buffer *empty_info;
+       struct sk_buff *empty;
+       void *data;
+
+       empty_info = &tx_ring->tx_buffer_info[tx_ring->next_to_use];
+       empty = alloc_skb(IGC_EMPTY_FRAME_SIZE, GFP_ATOMIC);
+       if (unlikely(!empty)) {
+               netdev_warn(tx_ring->netdev,
+                           "Fail to alloc skb for empty packet\n");
+               return;
+       }
+
+       data = skb_put(empty, IGC_EMPTY_FRAME_SIZE);
+       memset(data, 0, IGC_EMPTY_FRAME_SIZE);
+
+       /* Prepare DMA mapping and Tx buffer information */
+       if (unlikely(igc_init_empty_frame(tx_ring, empty_info, empty))) {
+               dev_kfree_skb_any(empty);
+               netdev_warn(tx_ring->netdev,
+                           "Fail to map DMA for empty packet\n");
+               return;
+       }
+
+       /* Prepare context descriptor for empty packet */
+       igc_tx_ctxtdesc(tx_ring, 0, false, 0, 0, 0);
+
+       /* Prepare data descriptor for empty packet */
+       igc_init_tx_empty_descriptor(tx_ring, empty, empty_info);
+}
+
 static netdev_tx_t igc_xmit_frame_ring(struct sk_buff *skb,
                                       struct igc_ring *tx_ring)
 {

Are above codes resolve your concern? If yes, i can test and add
them into this patch in next version submission.

Thanks & Regards
Siang