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

[Sven Van Asbroeck]

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?