Re: [PATCH net-next] net: mana: Linearize SKB if TX SGEs exceeds hardware limit

[Aditya Garg <gargaditya@linux.microsoft.com>]

On 04-10-2025 15:08, Simon Horman wrote:
> On Fri, Oct 03, 2025 at 08:47:24AM -0700, Aditya Garg wrote:
>> The MANA hardware supports a maximum of 30 scatter-gather entries (SGEs)
>> per TX WQE. In rare configurations where MAX_SKB_FRAGS + 2 exceeds this
>> limit, the driver drops the skb. Add a check in mana_start_xmit() to
>> detect such cases and linearize the SKB before transmission.
>>
>> Return NETDEV_TX_BUSY only for -ENOSPC from mana_gd_post_work_request(),
>> send other errors to free_sgl_ptr to free resources and record the tx
>> drop.
>>
>> Signed-off-by: Aditya Garg <gargaditya@linux.microsoft.com>
>> Reviewed-by: Dipayaan Roy <dipayanroy@linux.microsoft.com>
>> ---
>>   drivers/net/ethernet/microsoft/mana/mana_en.c | 26 +++++++++++++++----
>>   include/net/mana/gdma.h                       |  8 +++++-
>>   include/net/mana/mana.h                       |  1 +
>>   3 files changed, 29 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/microsoft/mana/mana_en.c b/drivers/net/ethernet/microsoft/mana/mana_en.c
>> index f4fc86f20213..22605753ca84 100644
>> --- a/drivers/net/ethernet/microsoft/mana/mana_en.c
>> +++ b/drivers/net/ethernet/microsoft/mana/mana_en.c
>> @@ -20,6 +20,7 @@
>>   
>>   #include <net/mana/mana.h>
>>   #include <net/mana/mana_auxiliary.h>
>> +#include <linux/skbuff.h>
>>   
>>   static DEFINE_IDA(mana_adev_ida);
>>   
>> @@ -289,6 +290,19 @@ 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;
>>   
>> +	BUILD_BUG_ON(MAX_TX_WQE_SGL_ENTRIES != MANA_MAX_TX_WQE_SGL_ENTRIES);
>> +	#if (MAX_SKB_FRAGS + 2 > MANA_MAX_TX_WQE_SGL_ENTRIES)
> 
> Hi Aditya,
> 
> I see that Eric has made a more substantial review of this patch,
> so please follow his advice.
> 
> But I wanted to add something to keep in mind for the future: I if the #if
> / #else used here can be replaced by a simple if() statement, then that
> would be preferable.  The advantage being that it improves compile
> coverage.  And, as these are all constants, I would expect the compiler to
> optimise away any unused code.

Hi Simon,
I will take care of yours and Eric's comment in v2 of this patch.
Regards,
Aditya