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

[Florian Fainelli <f.fainelli@gmail.com>]

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.

> 
>>> 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?
> 
> 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.

-- 
Florian