Low network throughput on i.MX28

[Stefan Wahren]

Hi Jörg,

> Jörg Krause <joerg.krause at embedded.rocks> hat am 4. November 2016 um 19:44
> geschrieben:
> 
> 
> Hi Shawn,
> 
> On Wed, 2016-11-02 at 09:24 +0100, Stefan Wahren wrote:
> > Am 02.11.2016 um 09:14 schrieb Jörg Krause:
> > > On Sat, 2016-10-29 at 11:08 +0200, Stefan Wahren wrote:
> > > > > Jörg Krause <joerg.krause at embedded.rocks> hat am 29. Oktober
> > > > > 2016
> > > > > um 01:07
> > > > > geschrieben:
> > > > > 
> > > > > 
> > > > > You mentioned [1] an optimization in the Freescale vendor Linux
> > > > > kernel
> > > > > [2]. I would really like to see this optimization in the
> > > > > mainline
> > > > > kernel.
> > > > > 
> > > > > Did you ever tried to port this code from Freescale to
> > > > > mainline?
> > > > 
> > > > Yes, i tried once but i was frustrated soon because of the lot of
> > > > required
> > > > changes and resulting issues.
> > > 
> > > I got the PIO mode working for the mxs-mmc driver. For this I
> > > ported
> > > the PIO code from the vendor kernel and removed the usage of the
> > > DMA
> > > engine entirely.
> > 
> > Good job
> > 
> > > 
> > > Testing network bandwidth with iperf, I get about ~10Mbit/sec with
> > > PIO
> > > mode compared to ~6.5Mbit/sec with DMA mode for UDP and about
> > > ~6.5Mbit/sec compared to ~4.5Mbit/sec with DMA mode for TCP.
> > 
> > And how about MMC / sd card performance?
> 
> I noticed poor performance with the i.MX28 MMC and/or DMA driver using
> the mainline kernel compared to the vendor Freescale kernel 2.6.35.
> I've seen that hou have added the drivers to mainline some years ago.
> 
> My custom i.MX28 board has a wifi chip attached to the SSP2 interface.
> Comparing the bandwith with iperf I get >20Mbits/sec on the vendor
> kernel and <5Mbits/sec on the mainline kernel.

there is one thing about the clock handling. I noticed that the Vendor Kernel
round up the clock frequency and the Mainline Kernel round down the clock
frequency [1]. So don't trust the clock ratings from DT / board code. Better
verify the register settings or check it with an osci.

[1] - http://www.spinics.net/lists/linux-mmc/msg09132.html

> 
> My best guess is that there is some kind of bottleneck in the drivers.
> I already started looking at the vendor drivers as well as at the
> mainline drivers, but I need some more investigation to understand the
> complexity.
> 
> Do you have any idea what the bottleneck might be?
> 
> Best regards,
> Jörg Krause