[PATCH iwl-net v2] igc: Reduce TSN TX packet buffer from 7KB to 5KB per queue

[Chwee-Lin Choong <chwee.lin.choong@intel.com>]

The previous 7 KB per queue caused TX unit hangs under heavy
timestamping load. Reducing to 5 KB avoids these hangs and matches
the TSN recommendation in I225/I226 SW User Manual Section 7.5.4.

The 8 KB “freed” by this change is currently unused. This reduction
is not expected to impact throughput, as the i226 is PCIe-limited
for small TSN packets rather than TX-buffer-limited.

Fixes: 0d58cdc902da ("igc: optimize TX packet buffer utilization for TSN mode")
Reported-by: Zdenek Bouska <zdenek.bouska@siemens.com>
Closes: https://lore.kernel.org/netdev/AS1PR10MB5675DBFE7CE5F2A9336ABFA4EBEAA@AS1PR10MB5675.EURPRD10.PROD.OUTLOOK.COM/
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Chwee-Lin Choong <chwee.lin.choong@intel.com>
---
v1: https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20250918183811.31270-1-chwee.lin.choong@intel.com/

changelog:
v1 -> v2 
- Elaborated commit message
---
 drivers/net/ethernet/intel/igc/igc_defines.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/intel/igc/igc_defines.h b/drivers/net/ethernet/intel/igc/igc_defines.h
index 498ba1522ca4..9482ab11f050 100644
--- a/drivers/net/ethernet/intel/igc/igc_defines.h
+++ b/drivers/net/ethernet/intel/igc/igc_defines.h
@@ -443,9 +443,10 @@
 #define IGC_TXPBSIZE_DEFAULT ( \
 	IGC_TXPB0SIZE(20) | IGC_TXPB1SIZE(0) | IGC_TXPB2SIZE(0) | \
 	IGC_TXPB3SIZE(0) | IGC_OS2BMCPBSIZE(4))
+/* TSN value following I225/I226 SW User Manual Section 7.5.4 */
 #define IGC_TXPBSIZE_TSN ( \
-	IGC_TXPB0SIZE(7) | IGC_TXPB1SIZE(7) | IGC_TXPB2SIZE(7) | \
-	IGC_TXPB3SIZE(7) | IGC_OS2BMCPBSIZE(4))
+	IGC_TXPB0SIZE(5) | IGC_TXPB1SIZE(5) | IGC_TXPB2SIZE(5) | \
+	IGC_TXPB3SIZE(5) | IGC_OS2BMCPBSIZE(4))
 
 #define IGC_DTXMXPKTSZ_TSN	0x19 /* 1600 bytes of max TX DMA packet size */
 #define IGC_DTXMXPKTSZ_DEFAULT	0x98 /* 9728-byte Jumbo frames */
-- 
2.43.0