linux-kernel - Re: [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB

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Message-ID: <16d1c66f-5451-2515-af73-a6b44d996e92@arm.com>
Date:   Thu, 18 Mar 2021 19:53:37 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Florian Fainelli <f.fainelli@...il.com>,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>
Cc:     Jonathan Corbet <corbet@....net>, opendmb@...il.com,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "open list:SWIOTLB SUBSYSTEM" <iommu@...ts.linux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB

On 2021-03-18 19:43, Florian Fainelli wrote:
> 
> 
> 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.

It also forces io_tlb_nslabs to the minimum, so it should be claiming 
considerably less than 64MB. IIRC the original proposal *did* skip 
initialisation completely, but that turned up the aforementioned issues.

>> 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@...il.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?

I wouldn't like it done to me personally, but for arm64, observe what 
mem_init() in arch/arm64/mm/init.c already does.

Robin.