Low network throughput on i.MX28

[Koul, Vinod]

On Sat, 2016-11-05 at 14:14 +0100, Jörg Krause wrote:
> On Sat, 2016-11-05 at 12:39 +0000, Koul, Vinod wrote:
> > 
> > On Sat, 2016-11-05 at 13:06 +0100, Jörg Krause wrote:
> > > 
> > > @ Vinod
> > > In short, I noticed poor performance in the SSP2 (MMC/SD/SDIO)
> > > interface on a custom i.MX28 board with a wifi chip attached.
> > > Comparing
> > > the bandwith with iperf I get >20Mbits/sec on the vendor kernel
> > > and
> > > <5Mbits/sec on the mainline kernel. I am trying to investigate
> > > what
> > > the
> > > bottleneck is.
> > is this imx-dma or imx-sdma..
> > 
> > > 
> > > 
> > > @ Stefan, all
> > > My understanding is that the tasklet in this case is responsible
> > > for
> > > reading the response registers of the DMA controller and return
> > > the
> > > response to the MMC host driver.
> > > 
> > > The vendor kernel does this in the interrupt routine of mxs-mmc by
> > > issueing a complete whereas the mainline kernel does this in the
> > > interrupt routine in mxs-dma by scheduling the tasklet.
> > Is vendor kernel using dmaengine APIs or not?
> It's this engine [1].
> 
> [1] http://git.freescale.com/git/cgit.cgi/imx/linux-2.6-imx.git/tree/a
> r
> ch/arm/plat-mxs/dmaengine.c?h=imx_2.6.35_1.1.0

Thanks for info, this looks okay.

First can you confirm that register configuration for DMA transaction is
same in both cases.

Second, looking at the driver I see that interrupt handler is not
pushing next descriptor. Also the tasklet is doing callback action and
not pushing any descriptors, did I miss anything in this?

For good dma throughput, you should have multiple dma transactions
queued up and submitted as fast as possible. Can you check if this is
being done. 

We need to minimize/eliminate the delay between two transactions. This
can be done in SW or HW based on support from HW. If HW supports
chaining of descriptors then next transaction which is given to
dmaengine driver should be appended at the end. If not submit the
descriptor to hw immediately on interrupt. 

For good example of latter please look at drivers/dma/sa11x0-dma.c

HTH
-- 
~Vinod