Re: [PATCH net-next v4 1/2] net: mana: Handle SKB if TX SGEs exceed hardware limit

Aditya Garg
2025-11-14T20:58:28Z

On 12-11-2025 19:25, Eric Dumazet wrote:
> On Wed, Nov 12, 2025 at 5:11 AM Aditya Garg
> <gargaditya@linux.microsoft.com> wrote:
>>
>> The MANA hardware supports a maximum of 30 scatter-gather entries (SGEs)
>> per TX WQE. Exceeding this limit can cause TX failures.
>> Add ndo_features_check() callback to validate SKB layout before
>> transmission. For GSO SKBs that would exceed the hardware SGE limit, clear
>> NETIF_F_GSO_MASK to enforce software segmentation in the stack.
>> Add a fallback in mana_start_xmit() to linearize non-GSO SKBs that still
>> exceed the SGE limit.
>>
>> Also, Add ethtool counter for SKBs linearized
>>
>> Co-developed-by: Dipayaan Roy <dipayanroy@linux.microsoft.com>
>> Signed-off-by: Dipayaan Roy <dipayanroy@linux.microsoft.com>
>> Signed-off-by: Aditya Garg <gargaditya@linux.microsoft.com>
>> ---
>> drivers/net/ethernet/microsoft/mana/mana_en.c | 37 ++++++++++++++++++-
>> .../ethernet/microsoft/mana/mana_ethtool.c | 2 +
>> include/net/mana/gdma.h | 6 ++-
>> include/net/mana/mana.h | 1 +
>> 4 files changed, 43 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/microsoft/mana/mana_en.c b/drivers/net/ethernet/microsoft/mana/mana_en.c
>> index cccd5b63cee6..67ae5421f9ee 100644
>> --- a/drivers/net/ethernet/microsoft/mana/mana_en.c
>> +++ b/drivers/net/ethernet/microsoft/mana/mana_en.c
>> @@ -11,6 +11,7 @@
>> #include <linux/mm.h>
>> #include <linux/pci.h>
>> #include <linux/export.h>
>> +#include <linux/skbuff.h>
>>
>> #include <net/checksum.h>
>> #include <net/ip6_checksum.h>
>> @@ -329,6 +330,20 @@ netdev_tx_t mana_start_xmit(struct sk_buff *skb, struct net_device *ndev)
>> cq = &apc->tx_qp[txq_idx].tx_cq;
>> tx_stats = &txq->stats;
>>
>> + if (MAX_SKB_FRAGS + 2 > MAX_TX_WQE_SGL_ENTRIES &&
>> + skb_shinfo(skb)->nr_frags + 2 > MAX_TX_WQE_SGL_ENTRIES) {
>> + /* GSO skb with Hardware SGE limit exceeded is not expected here
>> + * as they are handled in mana_features_check() callback
>> + */
>> + if (skb_linearize(skb)) {
>> + netdev_warn_once(ndev, "Failed to linearize skb with nr_frags=%d and is_gso=%d\n",
>> + skb_shinfo(skb)->nr_frags,
>> + skb_is_gso(skb));
>> + goto tx_drop_count;
>> + }
>> + apc->eth_stats.linear_pkt_tx_cnt++;
>> + }
>> +
>> pkg.tx_oob.s_oob.vcq_num = cq->gdma_id;
>> pkg.tx_oob.s_oob.vsq_frame = txq->vsq_frame;
>>
>> @@ -442,8 +457,6 @@ netdev_tx_t mana_start_xmit(struct sk_buff *skb, struct net_device *ndev)
>> }
>> }
>>
>> - WARN_ON_ONCE(pkg.wqe_req.num_sge > MAX_TX_WQE_SGL_ENTRIES);
>> -
>> if (pkg.wqe_req.num_sge <= ARRAY_SIZE(pkg.sgl_array)) {
>> pkg.wqe_req.sgl = pkg.sgl_array;
>> } else {
>> @@ -518,6 +531,25 @@ netdev_tx_t mana_start_xmit(struct sk_buff *skb, struct net_device *ndev)
>> return NETDEV_TX_OK;
>> }
>>
>
>
> #if MAX_SKB_FRAGS + 2 > MAX_TX_WQE_SGL_ENTRIES
>
>> +static netdev_features_t mana_features_check(struct sk_buff *skb,
>> + struct net_device *ndev,
>> + netdev_features_t features)
>> +{
>> + if (MAX_SKB_FRAGS + 2 > MAX_TX_WQE_SGL_ENTRIES &&
>> + skb_shinfo(skb)->nr_frags + 2 > MAX_TX_WQE_SGL_ENTRIES) {
>> + /* Exceeds HW SGE limit.
>> + * GSO case:
>> + * Disable GSO so the stack will software-segment the skb
>> + * into smaller skbs that fit the SGE budget.
>> + * Non-GSO case:
>> + * The xmit path will attempt skb_linearize() as a fallback.
>> + */
>> + if (skb_is_gso(skb))
>
> No need to test skb_is_gso(skb), you can clear bits, this will be a
> NOP if the packet is non GSO anyway.
>
>> + features &= ~NETIF_F_GSO_MASK;
>> + }
>> + return features;
>> +}
>
> #endif
>
>> +
>> static void mana_get_stats64(struct net_device *ndev,
>> struct rtnl_link_stats64 *st)
>> {
>> @@ -878,6 +910,7 @@ static const struct net_device_ops mana_devops = {
>> .ndo_open = mana_open,
>> .ndo_stop = mana_close,
>> .ndo_select_queue = mana_select_queue,
>> + .ndo_features_check = mana_features_check,
>
> Note that if your mana_features_check() is a nop if MAX_SKB_FRAGS is
> small enough,
> you could set a non NULL .ndo_features_check based on a preprocessor condition
>
> #if MAX_SKB_FRAGS + 2 > MAX_TX_WQE_SGL_ENTRIES
> .ndo_features_check = ....
> #endif
>
> This would avoid an expensive indirect call when possible.
>
>
>> .ndo_start_xmit = mana_start_xmit,
>> .ndo_validate_addr = eth_validate_addr,
>> .ndo_get_stats64 = mana_get_stats64,
>> diff --git a/drivers/net/ethernet/microsoft/mana/mana_ethtool.c b/drivers/net/ethernet/microsoft/mana/mana_ethtool.c
>> index a1afa75a9463..fa5e1a2f06a9 100644
>> --- a/drivers/net/ethernet/microsoft/mana/mana_ethtool.c
>> +++ b/drivers/net/ethernet/microsoft/mana/mana_ethtool.c
>> @@ -71,6 +71,8 @@ static const struct mana_stats_desc mana_eth_stats[] = {
>> {"tx_cq_err", offsetof(struct mana_ethtool_stats, tx_cqe_err)},
>> {"tx_cqe_unknown_type", offsetof(struct mana_ethtool_stats,
>> tx_cqe_unknown_type)},
>> + {"linear_pkt_tx_cnt", offsetof(struct mana_ethtool_stats,
>> + linear_pkt_tx_cnt)},
>> {"rx_coalesced_err", offsetof(struct mana_ethtool_stats,
>> rx_coalesced_err)},
>> {"rx_cqe_unknown_type", offsetof(struct mana_ethtool_stats,
>> diff --git a/include/net/mana/gdma.h b/include/net/mana/gdma.h
>> index 637f42485dba..84614ebe0f4c 100644
>> --- a/include/net/mana/gdma.h
>> +++ b/include/net/mana/gdma.h
>> @@ -592,6 +592,9 @@ enum {
>> #define GDMA_DRV_CAP_FLAG_1_HANDLE_RECONFIG_EQE BIT(17)
>> #define GDMA_DRV_CAP_FLAG_1_HW_VPORT_LINK_AWARE BIT(6)
>>
>> +/* Driver supports linearizing the skb when num_sge exceeds hardware limit */
>> +#define GDMA_DRV_CAP_FLAG_1_SKB_LINEARIZE BIT(20)
>> +
>> #define GDMA_DRV_CAP_FLAGS1 \
>> (GDMA_DRV_CAP_FLAG_1_EQ_SHARING_MULTI_VPORT | \
>> GDMA_DRV_CAP_FLAG_1_NAPI_WKDONE_FIX | \
>> @@ -601,7 +604,8 @@ enum {
>> GDMA_DRV_CAP_FLAG_1_DYNAMIC_IRQ_ALLOC_SUPPORT | \
>> GDMA_DRV_CAP_FLAG_1_SELF_RESET_ON_EQE | \
>> GDMA_DRV_CAP_FLAG_1_HANDLE_RECONFIG_EQE | \
>> - GDMA_DRV_CAP_FLAG_1_HW_VPORT_LINK_AWARE)
>> + GDMA_DRV_CAP_FLAG_1_HW_VPORT_LINK_AWARE | \
>> + GDMA_DRV_CAP_FLAG_1_SKB_LINEARIZE)
>>
>> #define GDMA_DRV_CAP_FLAGS2 0
>>
>> diff --git a/include/net/mana/mana.h b/include/net/mana/mana.h
>> index 8906901535f5..50a532fb30d6 100644
>> --- a/include/net/mana/mana.h
>> +++ b/include/net/mana/mana.h
>> @@ -404,6 +404,7 @@ struct mana_ethtool_stats {
>> u64 hc_tx_err_gdma;
>> u64 tx_cqe_err;
>> u64 tx_cqe_unknown_type;
>> + u64 linear_pkt_tx_cnt;
>> u64 rx_coalesced_err;
>> u64 rx_cqe_unknown_type;
>> };
>> --
>> 2.43.0
>>

Thanks for the review Eric. I will incorporate these changes in next
revision.

Regards,
Aditya