Low network throughput on i.MX28

[Stefan Wahren]

Hi Jörg,

> Jörg Krause <joerg.krause at embedded.rocks> hat am 4. November 2016 um 23:42
> geschrieben:
> 
> 
> Hi Stefan,
> 
> sorry, I forget the link in the previous mail.
> 
> On Fri, 2016-11-04 at 20:30 +0100, Stefan Wahren wrote:
> > 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
> 
> I checked the clock rate setting by reading the register 0x80014070
> (HW_SSP2_TIMING). CLOCK_DIVIDE is 0x2 and CLOCK_RATE is 0x0. As SSP CLK
> is 96MHz this makes a clock rate of 48MHz.
> 
> There was a discussion on the mailing list [1] about that tasklets
> might be slow.
> 
> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2011-February
> /043395.html

if i unterstand it right the tasklet is not the problem, but the design of the
MXS DMA driver. Please refer to the chapter "General Design Notes" to the
documentation of the DMA provider [2].
I think the MXS DMA driver is affected. Maybe you should ask Vinod Koul about
this.

[2] - https://www.kernel.org/doc/Documentation/dmaengine/provider.txt

> 
> Jörg