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

[Jakub Kicinski]

On Tue, 8 Dec 2020 18:02:30 -0500 Sven Van Asbroeck wrote:
> 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.

Good point!

> 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?

You can set DMA_ATTR_SKIP_CPU_SYNC and then sync only the part of the
buffer that got written.