Re: [PATCH v2 0/3] iommu: Enable non-strict DMA on QCom SD/MMC

From: Robin Murphy <robin.murphy@arm.com>
To: Joerg Roedel <joro@8bytes.org>, Doug Anderson <dianders@chromium.org>
Cc: Ulf Hansson <ulf.hansson@linaro.org>,
	Linux Doc Mailing List <linux-doc@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-pci@vger.kernel.org,
	Konrad Dybcio <konrad.dybcio@somainline.org>,
	Thierry Reding <thierry.reding@gmail.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Rajat Jain <rajatja@google.com>, Will Deacon <will@kernel.org>,
	Rob Clark <robdclark@chromium.org>,
	Saravana Kannan <saravanak@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	quic_c_gdjako@quicinc.com,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Veerabhadrarao Badiganti <vbadigan@codeaurora.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
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	Bjorn Helgaas <bhelgaas@google.com>,
	Sonny Rao <sonnyrao@chromium.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Randy Dunlap <rdunlap@infradead.org>,
	Linux MMC List <linux-mmc@vger.kernel.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	"list@263.net:IOMMU DRIVERS" <iommu@lists.linux-foundation.org>,
	Joerg Roedel <joro@8bytes.org>,
	iommu@lists.linux-foundation.org,
	Andrew Morton <akpm@linux-foundation.org>,
	"Maciej W. Rozycki" <macro@orcam.me.uk>
Subject: Re: [PATCH v2 0/3] iommu: Enable non-strict DMA on QCom SD/MMC
Date: Fri, 9 Jul 2021 14:56:47 +0100	[thread overview]
Message-ID: <edd1de35-5b9e-b679-9428-23c6d5005740@arm.com> (raw)
In-Reply-To: <YOaymBHc4g2cIfRn@8bytes.org>

On 2021-07-08 09:08, Joerg Roedel wrote:
> On Wed, Jul 07, 2021 at 01:00:13PM -0700, Doug Anderson wrote:
>> a) Nothing is inherently broken with my current approach.
>>
>> b) My current approach doesn't make anybody terribly upset even if
>> nobody is totally in love with it.
> 
> Well, no, sorry :)
> 
> I don't think it is a good idea to allow drivers to opt-out of the
> strict-setting. This is a platform or user decision, and the driver
> should accept whatever it gets.
> 
> So the real question is still why strict is the default setting and how
> to change that. Or document for the users that want performance how to
> change the setting, so that they can decide.

As I mentioned before, conceptually I think this very much belongs in 
sysfs as a user decision. We essentially have 4 levels of "strictness":

1: DMA domain with bounce pages
2: DMA domain
3: DMA domain with flush queue
4: Identity domain

The "make this device go faster because I trust it" use-case is why we 
have the sysfs interface to switch between 2 and 4, so it's entirely 
logical to have the intermediate option as well for when 3 is "faster" 
enough while still giving a bit more peace of mind than full-on bypass.

Making it a platform-specific decision that's hidden in a driver - 
arm-smmu-qcom can be considered a dumping ground of detailed platform 
knowledge ;) - happens to work as a reasonable compromise for this 
specific case, but I concur that it could be viewed as setting a 
precedent for other cases which we definitely aren't as reasonable.

I've been thinking about the sysfs thing some more, and since it's 
Friday afternoon and I can't concentrate on what I'm supposed to be 
doing anyway, let's see how far I can get by Monday...

Robin.