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

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Date:   Thu, 18 Mar 2021 17:48:12 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Robin Murphy <robin.murphy@....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 3/18/2021 4:35 PM, Robin Murphy wrote:
> On 2021-03-18 21:31, Florian Fainelli wrote:
>>
>>
>> On 3/18/2021 12:53 PM, Robin Murphy wrote:
>>> 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.
>>
>> AFAICT in that case we will have iotlb_n_slabs will set to 1, which will
>> still make us allocate io_tlb_n_slabs << IO_TLB_SHIFT bytes in
>> swiotlb_init(), which still gives us 64MB.
> 
> Eh? When did 2KB become 64MB? IO_TLB_SHIFT is 11, so that's at most one
> page in anyone's money...

Yes, sorry incorrect shift applied here. Still, and I believe this is
what you mean below, architecture code setting swiotlb_force =
SWIOTLB_NO_FORCE does not result in not allocating the SWIOTLB, because
io_tlb_nslabs is still left set to 0 so swiotlb_init() will proceed with
allocating the default size.

> 
>>>>> 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.
> 
> In fact I should have looked more closely at that myself - checking
> debugfs on my 4GB arm64 board actually shows io_tlb_nslabs = 0, and
> indeed we are bypassing initialisation completely and (ab)using
> SWIOTLB_NO_FORCE to cover it up, so I guess it probably *is* safe now
> for the noforce option to do the same for itself and save even that one
> page.

OK, I can submit a patch that does that. 5.12-rc3 works correctly for me
here as well and only allocates SWIOTLB when needed which in our case is
either:

- we have DRAM at PA >= 4GB
- we have limited peripherals (Raspberry Pi 4 derivative) that can only
address the lower 1GB

Now let's see if we can get ARM 32-bit to match :)
-- 
Florian