Re: [PATCH net-next 08/13] net: macb: introduce DMA descriptor helpers (is 64bit? is PTP?)

[Maxime Chevallier]

Hello Théo,

On Fri, 21 Mar 2025 20:09:39 +0100
Théo Lebrun <theo.lebrun@xxxxxxxxxxx> wrote:

> Introduce macb_dma_is_64b() and macb_dma_is_ptp() helper functions.
> Many codepaths are made simpler by dropping conditional compilation.
> 
> This implies three changes:
>  - Always compile related structure definitions inside <macb.h>.
>  - Make the field hw_dma_cap in struct macb always present.
>  - MACB_EXT_DESC can be dropped as it is useless now.
> 
> The common case is:
> 
> 	#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
> 		struct macb_dma_desc_64 *desc_64;
> 		if (bp->hw_dma_cap & HW_DMA_CAP_64B) {
> 			desc_64 = macb_64b_desc(bp, desc);
> 			// ...
> 		}
> 	#endif
> 
> And replaced by:
> 
> 	struct macb_dma_desc_64 *desc_64;
> 	if (macb_dma_is_64b(bp)) {
> 		desc_64 = macb_64b_desc(bp, desc);
> 		// ...
> 	}

Just a thought, but this is adding some more branches in the hotpath on
32 bits DMA setups. Did you measure any performance changes on
these platforms (if you have access to one :) )

As the caps can't be changed dynamically, maybe these helpers could be
replaced more efficiently with some static_key ? This would benefit
both 32 and 64 bits systems as the following would be more efficient

	if (bp->hw_dma_cap & HW_DMA_CAP_64B) {
		//  ...
	}

Just a thought of course, maybe this patch doesn't really hurt perfs :)

Maxime