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

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Message-ID: <df8f1bf4-819a-4b2a-927d-e97fe196cdf6@arm.com>
Date: Mon, 29 Jan 2024 12:15:07 +0000
From: Robin Murphy <robin.murphy@....com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>,
 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>, 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

On 2024-01-29 11:07 am, Alexander Lobakin wrote:
> 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.

In fact those wrappers could perhaps subsume the existing stub 
definitions, by starting with a refactor along these lines:

static inline dma_sync_x(...)
{
	if (IS_ENABLED(CONFIG_NEED_DMA_SYNC))
		__dma_sync_x(...);
}

Cheers,
Robin.