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Date: Mon, 29 Jan 2024 07:09:47 +0100
From: Christoph Hellwig <hch@lst.de>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Petr =?utf-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>,
	Alexander Lobakin <aleksander.lobakin@intel.com>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Christoph Hellwig <hch@lst.de>,
	Marek Szyprowski <m.szyprowski@samsung.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
Subject: Re: [PATCH net-next 2/7] dma: avoid expensive redundant calls for
 sync operations
Message-ID: <20240129060947.GC19258@lst.de>
References: <20240126135456.704351-1-aleksander.lobakin@intel.com> <20240126135456.704351-3-aleksander.lobakin@intel.com> <0f6f550c-3eee-46dc-8c42-baceaa237610@arm.com> <7ff3cf5d-b3ff-4b52-9031-30a1cb71c0c9@intel.com> <0cf72c00-21d9-4f1a-be14-80336da5dff4@arm.com> <20240126194819.147cb4e2@meshulam.tesarici.cz> <1c62d388-a600-40d8-b386-15841cb1af95@arm.com>
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On Fri, Jan 26, 2024 at 07:13:05PM +0000, Robin Murphy wrote:
>> Can we have a comment that states this assumption along with the flag?
>> Because when it breaks, it will keep someone cursing for days why DMA
>> sometimes fails on their device before they find out it's not synced.
>> And then wondering why the code makes such silly assumptions...
>
> Indeed, apologies if it wasn't totally clear, but I really was implying a 
> literal "may skip sync if coherent and not using SWIOTLB (which matches 
> dma-direct)" flag, documented as such, and not trying to dress it up as 
> anything more generic. I just can't suggest a suitably concise name for 
> that of the top of my head... :)

Yes, that seems like the right way to go.