On Fri, Mar 21, 2025 at 08:09:38PM +0100, Théo Lebrun wrote:
The controller does IP alignment (two bytes).
I'm a bit confused here. Is this hard coded, baked into the silicon?
It will always do IP alignment? It cannot be turned off?
skb_reserve(skb, NET_IP_ALIGN);
Why not just replace this with
skb_reserve(skb, 2);
The NET_IP_ALIGN value is arch-dependent and picked based on unaligned
CPU access performance. The hardware alignment value should be
compatible-specific rather than arch-specific. Offer a path forward by
adding a hw_ip_align field inside macb_config.
Values for macb_config->hw_ip_align are picked based on upstream
devicetrees:
Compatible | DTS folders | hw_ip_align
------------------------|---------------------------|----------------
cdns,at91sam9260-macb | arch/arm/ | 2
cdns,macb | arch/{arm,riscv}/ | NET_IP_ALIGN
cdns,np4-macb | NULL | NET_IP_ALIGN
cdns,pc302-gem | NULL | NET_IP_ALIGN
cdns,gem | arch/{arm,arm64}/ | NET_IP_ALIGN
cdns,sam9x60-macb | arch/arm/ | 2
atmel,sama5d2-gem | arch/arm/ | 2
atmel,sama5d29-gem | arch/arm/ | 2
atmel,sama5d3-gem | arch/arm/ | 2
atmel,sama5d3-macb | arch/arm/ | 2
atmel,sama5d4-gem | arch/arm/ | 2
cdns,at91rm9200-emac | arch/arm/ | 2
cdns,emac | arch/arm/ | 2
cdns,zynqmp-gem | *same as xlnx,zynqmp-gem* | 0
cdns,zynq-gem | *same as xlnx,zynq-gem* | 2
sifive,fu540-c000-gem | arch/riscv/ | 2
microchip,mpfs-macb | arch/riscv/ | 2
microchip,sama7g5-gem | arch/arm/ | 2
microchip,sama7g5-emac | arch/arm/ | 2
xlnx,zynqmp-gem | arch/arm64/ | 0
xlnx,zynq-gem | arch/arm/ | 2
xlnx,versal-gem | NULL | NET_IP_ALIGN
I don't remember seeing any other driver doing anything like
this. That often means it is wrong....
Andrew