linux-kernel - Re: [PATCH net-next 1/7] dma: compile-out DMA sync op calls when not used

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Date: Mon, 29 Jan 2024 12:07:30 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Christoph Hellwig <hch@....de>
CC: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Marek Szyprowski <m.szyprowski@...sung.com>, "Robin
 Murphy" <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>, Will Deacon
	<will@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael
 J. Wysocki" <rafael@...nel.org>, Magnus Karlsson <magnus.karlsson@...el.com>,
	Maciej Fijalkowski <maciej.fijalkowski@...el.com>, Alexander Duyck
	<alexanderduyck@...com>, <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
	<iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 1/7] dma: compile-out DMA sync op calls when not
 used

From: Christoph Hellwig <hch@....de>
Date: Mon, 29 Jan 2024 07:11:36 +0100

> On Fri, Jan 26, 2024 at 02:54:50PM +0100, Alexander Lobakin wrote:
>> Some platforms do have DMA, but DMA there is always direct and coherent.
>> Currently, even on such platforms DMA sync operations are compiled and
>> called.
>> Add a new hidden Kconfig symbol, DMA_NEED_SYNC, and set it only when
>> either sync operations are needed or there is DMA ops or swiotlb
>> enabled. Set dma_need_sync() and dma_skip_sync() (stub for now)
>> depending on this symbol state and don't call sync ops when
>> dma_skip_sync() is true.
>> The change allows for future optimizations of DMA sync calls depending
>> on compile-time or runtime conditions.
> 
> So the idea of compiling out the calls sounds fine to me.  But what
> is the point of the extra indirection through the __-prefixed calls?

Because dma_sync_* ops are external functions, not inlines, and in the
next patch I'm adding a check there.

> 
> And if we need that (please document it in the commit log), please
> make the wrappers proper inline functions and not macros.

Thanks,
Olek