igc: optimize TX packet buffer utilization for TSN mode

In preparation for upcoming frame preemption patches, optimize the TX
packet buffer size. The total packet buffer size (RX + TX) is 64KB, with
a maximum of 34KB for either RX or TX. Split the buffer evenly,
allocating 32KB to each.

For TX, assign 7KB to each of the four TX packet buffers (total 28KB)
and reserve 4KB for BMC.

References:
I225/I226 SW User Manual Section 4.7.9, Section 8.3.2

Co-developed-by: Vinicius Costa Gomes <[email protected]>
Signed-off-by: Vinicius Costa Gomes <[email protected]>
Signed-off-by: Faizal Rahim <[email protected]>
Tested-by: Mor Bar-Gabay <[email protected]>
Signed-off-by: Tony Nguyen <[email protected]>

Author: Faizal Rahim

drivers/net/ethernet/intel/igc/igc_defines.h

@@ -419,8 +419,8 @@
 	IGC_TXPB0SIZE(20) | IGC_TXPB1SIZE(0) | IGC_TXPB2SIZE(0) | \
 	IGC_TXPB3SIZE(0) | IGC_OS2BMCPBSIZE(4))
 #define IGC_TXPBSIZE_TSN ( \
-	IGC_TXPB0SIZE(5) | IGC_TXPB1SIZE(5) | IGC_TXPB2SIZE(5) | \
-	IGC_TXPB3SIZE(5) | IGC_OS2BMCPBSIZE(4))
+	IGC_TXPB0SIZE(7) | IGC_TXPB1SIZE(7) | IGC_TXPB2SIZE(7) | \
+	IGC_TXPB3SIZE(7) | 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 */