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Subject: Re: [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB
To:     Robin Murphy <robin.murphy@arm.com>, linux-kernel@vger.kernel.org,
        Christoph Hellwig <hch@lst.de>
Cc:     opendmb@gmail.com, Jonathan Corbet <corbet@lwn.net>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        "open list:SWIOTLB SUBSYSTEM" <iommu@lists.linux-foundation.org>
References: <20210318191816.4185226-1-f.fainelli@gmail.com>
 <bbd44c42-cedc-7bd6-a443-c991fd080298@gmail.com>
 <e7850feb-b7cd-e279-e3fc-a9bdba162423@arm.com>
From:   Florian Fainelli <f.fainelli@gmail.com>
Message-ID: <f0940ea7-79f6-af57-aa7d-d121abb99012@gmail.com>
Date:   Thu, 18 Mar 2021 12:43:09 -0700
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On 3/18/2021 12:34 PM, Robin Murphy wrote:
> On 2021-03-18 19:22, Florian Fainelli wrote:
>>
>>
>> On 3/18/2021 12:18 PM, Florian Fainelli wrote:
>>> It may be useful to disable the SWIOTLB completely for testing or when a
>>> platform is known not to have any DRAM addressing limitations what so
>>> ever.
> 
> Isn't that what "swiotlb=noforce" is for? If you're confident that we've
> really ironed out *all* the awkward corners that used to blow up if
> various internal bits were left uninitialised, then it would make sense
> to just tweak the implementation of what we already have.

swiotlb=noforce does prevent dma_direct_map_page() from resorting to the
swiotlb, however what I am also after is reclaiming these 64MB of
default SWIOTLB bounce buffering memory because my systems run with
large amounts of reserved memory into ZONE_MOVABLE and everything in
ZONE_NORMAL is precious at that point.

> 
> I wouldn't necessarily disagree with adding "off" as an additional alias
> for "noforce", though, since it does come across as a bit wacky for
> general use.
> 
>>> Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
>>
>> Christoph, in addition to this change, how would you feel if we
>> qualified the swiotlb_init() in arch/arm/mm/init.c with a:
>>
>>
>> if (memblock_end_of_DRAM() >= SZ_4G)
>>     swiotlb_init(1)
> 
> Modulo "swiotlb=force", of course ;)

Indeed, we would need to handle that case as well. Does it sound
reasonable to do that to you as well?
-- 
Florian