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Date: Mon, 29 Jan 2024 12:07:30 +0100
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Subject: Re: [PATCH net-next 1/7] dma: compile-out DMA sync op calls when not
 used
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To: Christoph Hellwig <hch@lst.de>
CC: "David S. Miller" <davem@davemloft.net>, Eric Dumazet
	<edumazet@google.com>, Jakub Kicinski <kuba@kernel.org>, Paolo Abeni
	<pabeni@redhat.com>, Marek Szyprowski <m.szyprowski@samsung.com>, "Robin
 Murphy" <robin.murphy@arm.com>, Joerg Roedel <joro@8bytes.org>, Will Deacon
	<will@kernel.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, "Rafael
 J. Wysocki" <rafael@kernel.org>, Magnus Karlsson <magnus.karlsson@intel.com>,
	Maciej Fijalkowski <maciej.fijalkowski@intel.com>, Alexander Duyck
	<alexanderduyck@fb.com>, <bpf@vger.kernel.org>, <netdev@vger.kernel.org>,
	<iommu@lists.linux.dev>, <linux-kernel@vger.kernel.org>
References: <20240126135456.704351-1-aleksander.lobakin@intel.com>
 <20240126135456.704351-2-aleksander.lobakin@intel.com>
 <20240129061136.GD19258@lst.de>
From: Alexander Lobakin <aleksander.lobakin@intel.com>
In-Reply-To: <20240129061136.GD19258@lst.de>
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From: Christoph Hellwig <hch@lst.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