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

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Message-ID: <20250324095522.2ab1c38b@fedora.home>
Date: Mon, 24 Mar 2025 09:55:22 +0100
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: Théo Lebrun <theo.lebrun@...tlin.com>
Cc: Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
 <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Rob Herring
 <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Nicolas Ferre <nicolas.ferre@...rochip.com>, Claudiu
 Beznea <claudiu.beznea@...on.dev>, Paul Walmsley
 <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou
 <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>, Samuel Holland
 <samuel.holland@...ive.com>, Richard Cochran <richardcochran@...il.com>,
 Russell King <linux@...linux.org.uk>, Thomas Bogendoerfer
 <tsbogend@...ha.franken.de>, Vladimir Kondratiev
 <vladimir.kondratiev@...ileye.com>, Gregory CLEMENT
 <gregory.clement@...tlin.com>, netdev@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-riscv@...ts.infradead.org, linux-mips@...r.kernel.org, Thomas
 Petazzoni <thomas.petazzoni@...tlin.com>, Tawfik Bayouk
 <tawfik.bayouk@...ileye.com>
Subject: Re: [PATCH net-next 08/13] net: macb: introduce DMA descriptor
 helpers (is 64bit? is PTP?)

Hello Théo,

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