Re: [PATCH net v1 2/2] lan743x: boost performance: limit PCIe bandwidth requirement

[Florian Fainelli]

On 12/8/20 3:02 PM, Sven Van Asbroeck wrote:
> Hi Andrew,
> 
> On Tue, Dec 8, 2020 at 5:51 PM Andrew Lunn <andrew@xxxxxxx> wrote:
>>
>>>
>>> So I assumed that it's a PCIe dma bandwidth issue, but I could be wrong -
>>> I didn't do any PCIe bandwidth measurements.
>>
>> Sometimes it is actually cache operations which take all the
>> time. This needs to invalidate the cache, so that when the memory is
>> then accessed, it get fetched from RAM. On SMP machines, cache
>> invalidation can be expensive, due to all the cross CPU operations.
>> I've actually got better performance by building a UP kernel on some
>> low core count ARM CPUs.
>>
>> There are some tricks which can be played. Do you actually need all
>> 9K? Does the descriptor tell you actually how much is used? You can
>> get a nice speed up if you just unmap 64 bytes for a TCP ACK, rather
>> than the full 9K.
>>
> 
> Thank you for the suggestion! The original driver developer chose 9K because
> presumably that's the largest frame size supported by the chip.
> 
> Yes, I believe the chip will tell us via the descriptor how much it has
> written, I would have to double-check. I was already looking for a
> "trick" to transfer only the required number of bytes, but I was led to
> believe that dma_map_single() and dma_unmap_single() always needed to match.
> 
> So:
> dma_map_single(9K) followed by dma_unmap_single(9K) is correct, and
> dma_map_single(9K) followed by dma_unmap_single(1500 bytes) means trouble.
> 
> How can we get around that?

dma_sync_single_for_{cpu,device} is what you would need in order to make
a partial cache line invalidation. You would still need to unmap the
same address+length pair that was used for the initial mapping otherwise
the DMA-API debugging will rightfully complain.
-- 
Florian