[U-Boot] [PATCH v3 9/9] net/designware: Reduce DMA burst length

[Marek Vasut]

On Monday, April 28, 2014 at 07:51:49 AM, Chen-Yu Tsai wrote:
> On Mon, Apr 28, 2014 at 2:08 AM, Marek Vasut <marex at denx.de> wrote:
> > On Sunday, April 27, 2014 at 05:29:29 PM, Chen-Yu Tsai wrote:
> >> On Sun, Apr 27, 2014 at 11:25 PM, Ian Campbell <ijc at hellion.org.uk> wrote:
> >> > On Sat, 2014-04-26 at 20:28 +0200, Marek Vasut wrote:
> >> >> On Friday, April 18, 2014 at 08:05:50 PM, Ian Campbell wrote:
> >> >> > From: Jens Kuske <jenskuske at gmail.com>
> >> >> > 
> >> >> > The GMAC module in Allwinner sunxi SoCs seems to have problems with
> >> >> > burst lengths > 8.
> >> >> 
> >> >> Is there any explanation for the problems please ?
> >> > 
> >> > Jens or Wens, can you answer this?
> >> 
> >> IIRC, with burst lengths > 8, GMAC doesn't work, no ping, no DHCP.
> >> I don't remember if it was TX or RX that suffered, or even both.
> >> 
> >> Hope this clarifies things a bit.
> > 
> > No, it does not at all, sorry. What you describe are symptoms, but what I
> > want to know is what is the root cause of those symptoms. You did not
> > explain that.
> 
> I can not offer much more explanation. The value was hardcoded in
> Allwinner's code. The datasheets don't offer much, except this line
> might be related:
> 
>   (DMA) Descriptor architecture, allowing large blocks of data transfer
>   with minimum CPU intervention; each descriptor can transfer up to
>   4 KB data.
> 
> Also probably related:
> 
>   4KB TX FIFO for transmission packets and 16KB RX FIFO for reception
>   packets.
> 
> I'm not an expert in hardware. We could ask Allwinner, but given past
> inquiries from the linux-sunxi community, I'd say getting a reply on
> hardware specifics is unlikely to happen.
> 
> So my guess is that this is limited by the DWMAC IP Allwinner licensed
> from Synopsys.

+CC Alexey

Best regards,
Marek Vasut