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

Maxime Chevallier
2025-03-24T08:55:41Z

Hello Théo,

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