Re: [PATCH 1/2] drm/prime: Iterate SG DMA addresses separately

[okaya-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org]

On 2018-04-12 06:33, Christian König wrote:
> Am 12.04.2018 um 11:49 schrieb Lucas Stach:
>> Am Donnerstag, den 12.04.2018, 11:35 +0200 schrieb Christian König:
>>> Am 12.04.2018 um 11:11 schrieb Lucas Stach:
>>>> Am Mittwoch, den 11.04.2018, 20:26 +0200 schrieb Christian König:
>>>>> Am 11.04.2018 um 19:11 schrieb Robin Murphy:
>>>>>> For dma_map_sg(), DMA API implementations are free to merge 
>>>>>> consecutive
>>>>>> segments into a single DMA mapping if conditions are suitable, 
>>>>>> thus the
>>>>>> resulting DMA addresses may be packed into fewer entries than
>>>>>> ttm->sg->nents implies.
>>>>>> 
>>>>>> drm_prime_sg_to_page_addr_arrays() does not account for this, 
>>>>>> meaning
>>>>>> its callers either have to reject the 0 < count < nents case or 
>>>>>> risk
>>>>>> getting bogus addresses back later. Fortunately this is relatively 
>>>>>> easy
>>>>>> to deal with having to rejig structures to also store the mapped 
>>>>>> count,
>>>>>> since the total DMA length should still be equal to the total 
>>>>>> buffer
>>>>>> length. All we need is a separate scatterlist cursor to iterate 
>>>>>> the DMA
>>>>>> addresses separately from the CPU addresses.
>>>>> Mhm, I think I like Sinas approach better.
>>>>> 
>>>>> See the hardware actually needs the dma_address on a page by page 
>>>>> basis.
>>>>> 
>>>>> Joining multiple consecutive pages into one entry is just 
>>>>> additional
>>>>> overhead which we don't need.
>>>> But setting MAX_SEGMENT_SIZE will probably prevent an IOMMU that 
>>>> might
>>>> be in front of your GPU to map large pages as such, causing 
>>>> additional
>>>> overhead by means of additional TLB misses and page walks on the 
>>>> IOMMU
>>>> side.
>>>> 
>>>> And wouldn't you like to use huge pages at the GPU side, if the 
>>>> IOMMU
>>>> already provides you the service of producing one large consecutive
>>>> address range, rather than mapping them via a number of small pages?
>>> No, I wouldn't like to use that. We're already using that :)
>>> 
>>> But we use huge pages by allocating consecutive chunks of memory so 
>>> that
>>> both the CPU as well as the GPU can increase their TLB hit rate.
>> I'm not convinced that this is a universal win. Many GPU buffers 
>> aren't
>> accessed by the CPU and allocating huge pages puts much more strain on
>> the kernel MM subsystem.
> 
> Yeah, we indeed see the extra overhead during allocation.
> 
>>> What currently happens is that the DMA subsystem tries to coalesce
>>> multiple pages into on SG entry and we de-coalesce that inside the
>>> driver again to create our random access array.
>> I guess the right thing would be to have a flag that tells the the DMA
>> implementation to not coalesce the entries. But (ab-)using max segment
>> size tells the DMA API to split the segments if they are larger than
>> the given size, which is probably not what you want either as you now
>> need to coalesce the segments again when you are mapping real huge
>> pages.
> 
> No, even with huge pages I need an array with every single dma address
> for a 4K pages (or whatever the normal page size is).
> 
> The problem is that I need a random access array for the DMA
> addresses, even when they are continuously.
> 
> I agree that max segment size is a bit ugly here, but at least for
> radeon, amdgpu and pretty much TTM in general it is exactly what I
> need.
> 
> I could fix TTM to not need that, but for radeon and amdgpu it is the
> hardware which needs this.

I can implement i915 approach as Robin suggested as an alternative. Is 
that better?


> 
> Christian.
> 
>> 
>>> That is a huge waste of memory and CPU cycles and I actually wanted 
>>> to
>>> get rid of it for quite some time now. Sinas approach seems to be a 
>>> good
>>> step into the right direction to me to actually clean that up.
>>> 
>>>> Detecting such a condition is much easier if the DMA map 
>>>> implementation
>>>> gives you the coalesced scatter entries.
>>> A way which preserves both path would be indeed nice to have, but 
>>> that
>>> only allows for the TLB optimization for the GPU and not the CPU any
>>> more. So I actually see that as really minor use case.
>> Some of the Tegras have much larger TLBs and better page-walk
>> performance on the system IOMMU compared to the GPU MMU, so they would
>> probably benefit a good deal even if the hugepage optimization only
>> targets the GPU side.
>> 
>> Regards,
>> Lucas
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