net: stmmac: add attribute to disable split header support

Split headers (SPH) is a DMA feature to split the header and payload in
the receive path. See `SPH` bit in `EMAC_DMA[n]_CTL` register on page
30-255 of the ADSP-SC598 HRM.

With this feature enabled I ran into a oops fairly quickly on 5.15 and
immediately on 6.12.

  [ 6129.476843] Unable to handle kernel paging request at virtual address ffff00019414af80
  ...
  [ 6129.657171]  dcache_inval_poc+0x28/0x58
  [ 6129.660990]  dma_direct_sync_single_for_cpu+0x3c/0x70
  [ 6129.666024]  __dma_sync_single_for_cpu+0x34/0x6c
  [ 6129.670625]  stmmac_napi_poll_rx+0x8dc/0xb64
  ...

https://lore.kernel.org/all/CABnpCuCLN6VNgmoWHwc4_8AT34xqmQnEoUHLncvE2yLqYZBaKg@mail.gmail.com/

Matches the above stack trace most closely and includes some debugging
information.

https://lore.kernel.org/all/[email protected]

Includes some debug code from an Nvidia Tegra developer who states the
code, "causes other issues". In my testing passing `buf1_len` causes
the following:

  Connection to ... closed by remote host.

Note that `dwmac-intel.c` and `dwmac-dwc-qos-eth.c`, primarily used by
Nvidia Tegra SoCs, have already disabled this feature:

  47f753c ("net: stmmac: disable Split Header (SPH) for Intel platforms")
  029c1c2 ("net: stmmac: dwc-qos: Disable split header for Tegra194")

In testing disabling split headers resulted in a 200 Mbits/sec decrease
in performance on receive.

Signed-off-by: Philip Molloy <[email protected]>

Author: pamolloy

drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c

@@ -503,6 +503,9 @@ stmmac_probe_config_dt(struct platform_device *pdev, u8 *mac)
 	if (of_property_read_bool(np, "snps,en-tx-lpi-clockgating"))
 		plat->flags |= STMMAC_FLAG_EN_TX_LPI_CLOCKGATING;
 
+	if (of_property_read_bool(np, "snps,sph-disable"))
+		plat->flags |= STMMAC_FLAG_SPH_DISABLE;
+
 	/* Set the maxmtu to a default of JUMBO_LEN in case the
 	 * parameter is not present in the device tree.
 	 */