Subject: Low network throughput on i.MX28

From: [email-masked] (Jörg Krause)
Date: Sat, 19 Nov 2016 00:49:01 +0100

Hi all,

[snip]

I did some time measurements on the wifi, mmc and dma driver to compare
the performance between the vendor and the mainline kernel. For this I
toggled some GPIOs and measured the time difference with an osci. I
started measuring the time before calling sdio_readsb() in the wifi
driver [1] and stopped the time when the call returns. Note that the
time was only measured for a packet length of 1536 bytes.

The vendor kernel took about 250 us to return whereas the mainline
kernel took about 325 us. To investigate where this additional time
comes from I divided the whole procedure into seperate parts and
compared their time consumed.

I noticed that the mainline kernel does took much longer to return
after the DMA request is done, signalled in this case by calling
mxs_mmc_dma_irq_callback() [2] in the mxs-mmc driver. From here it
takes about 150 us to get back to sdio_readsb().

An example for consuming much more time is the mainline mmc driver
where it hangs in?mmc_wait_done() [2] about 50 us just calling
complete(), whereas the vendor mmc driver almost immediately returns
here.

I wonder why this call to complete consumes so much time? Any ideas?

[1] http://lxr.free-electrons.com/source/drivers/net/wireless/broadcom/
brcm80211/brcmfmac/bcmsdh.c#L488

[2] http://lxr.free-electrons.com/source/drivers/mmc/host/mxs-mmc.c#L17
9

[3] http://lxr.free-electrons.com/source/drivers/mmc/core/core.c#L386

Best regards,
J?rg Krause