Re: [PATCH net-next v2 2/2] net: mvneta: Use cacheable memory to store the rx buffer DMA address

From: Thomas Petazzoni
Date: Fri Feb 17 2017 - 08:55:20 EST


On Fri, 17 Feb 2017 14:30:03 +0100, Gregory CLEMENT wrote:

> I have just tested it and as I feared, with HWBM enabled, a simple iperf
> just doesn't work.

And that's expected: the whole point of HWBM is that the buffer into
which a RX packet is placed is allocated by the HW, and its address
stored in the RX descriptor. So the following code:

> > rx_desc->buf_phys_addr = phys_addr;
> > i = rx_desc - rxq->descs;
> > + rxq->buf_dma_addr[i] = phys_addr;

Does not make sense, because it's not the SW that refills the RX
descriptors with the address of the RX buffers. It's done by the HW.

With HWBM, I believe you have no choice but to read the physical
address from the RX descriptor. But you can probably optimize things a
little bit by reading it only once, and then storing it into a
cacheable variable.

So maybe:

- For SWBM, use the strategy proposed by Jisheng
- For HWBM, at the beginning of the RX completion path, read once the
rx_desc->buf_phys_addr, and store it in rxq->buf_dma_addr[index]

Of course that's just a very rough proposal. I've been looking mainly
at mvpp2 lately, and I'm not sure I still remember how mvneta works in
the details.

Best regards,

Thomas Petazzoni, CTO, Free Electrons
Embedded Linux and Kernel engineering