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

Hello Maxime,

On Mon Mar 24, 2025 at 9:55 AM CET, Maxime Chevallier wrote:

On Fri, 21 Mar 2025 20:09:39 +0100
Théo Lebrun [off-list ref] wrote:

quoted

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 :)

Good question! I asked myself the same thing before posting.

We go from:

	void bar(struct macb *bp) {
	#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
		if (bp->hw_dma_cap & HW_DMA_CAP_64B) {
			foo();
		}
	#endif
	}

To:

	static bool macb_dma_is_64b(struct macb *bp)
	{
		return IS_ENABLED(CONFIG_ARCH_DMA_ADDR_T_64BIT) &&
		       bp->hw_dma_cap & HW_DMA_CAP_64B;
	}

	void bar(struct macb *bp) {
		if (macb_dma_is_64b(bp)) {
			foo();
		}
	}

In the first case, we use explicit preprocessor directives to remove
code if CONFIG_ARCH_DMA_ADDR_T_64BIT isn't defined.

In the second case, our compiler optimises away the IS_ENABLED() call.
 - If false, then the branch doesn't appear.
 - If true, then only the capability check is inlined in bar().
I checked the assembly on arm/arm64/MIPS.

Conclusion: the hotpath doesn't change.

Thanks,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com