netdev - Re: [PATCH net-next 2/7] dma: avoid expensive redundant calls for sync operations

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Date: Mon, 29 Jan 2024 07:09:47 +0100
From: Christoph Hellwig <hch@....de>
To: Robin Murphy <robin.murphy@....com>
Cc: Petr Tesařík <petr@...arici.cz>,
	Alexander Lobakin <aleksander.lobakin@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Christoph Hellwig <hch@....de>,
	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 2/7] dma: avoid expensive redundant calls for
 sync operations

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.