NSLU2 dma_free_coherent issue with DIB0700 driver (and probably others)

Hi Lee,
I was browsing the mailing-list archive and found your email about the issue with DIB0700-based devices on an NSLU2. I myself use a Terratec Cinergy DT XS Diversity which I think had the same issue as you described it. I found that the cause was the rather big DMA buffer of 39480 bytes as specified in dib0700_devices.c.
Looking at the kernel's arch/arm/mach-ixp4xx/common-pci.c there is a top limit of 4096 bytes for a DMA buffer set by the function call
dmabounce_register_dev(dev, 2048, 4096);

My recipe (for SlugOS) below changes the big buffer to a smaller one and also increases the number of buffers (not sure if the latter is really needed). Now the module is working just fine, even recording on both adapters gives a CPU usage of just ~25%. So the smaller buffer size does not seem to hurt at all. Not sure why it is needed in the first place.

Bye,
-Thomas

PS: My recipe "v4l-dvb_0776e4801991.bb"

========= CUT ==========

DESCRIPTION = "v4l-dvb modules"
#SECTION = ""
PRIORITY = "optional"
HOMEPAGE = "http://linuxtv.org/"
LICENSE = "GPL"
DEPENDS = "virtual/kernel"
PR = "r0"

SRC_URI = "http://linuxtv.org/hg/v4l-dvb/archive/${PV}.tar.bz2"

S = "${WORKDIR}/${PN}-${PV}"

inherit module

MAKE_TARGETS = "DIR=${STAGING_KERNEL_DIR}"

do_configure() {

	# fix make target
	cd "${S}"
	sed -i 's%^install:%install modules_install:%' Makefile

	# reduce buffer size (ixp4xx can handle only <= 4096
	# (see arch/arm/mach-ixp4xx/common-pci.c)
	cd "${S}/linux/drivers/media/dvb/dvb-usb"
	sed -i 's%buffersize = 39480%buffersize = 4096%' dib0700_devices.c
	sed -i 's%\.count = 4,%.count = 7,%' dib0700_devices.c

	# do not strip here
	cd "${S}/v4l/scripts"
	sed -i 's%@strip %@#strip %' make_makefile.pl
}

fakeroot do_install() {
	oe_runmake DESTDIR="${D}" install
}

FILES_${PN} = "/lib/modules"

========= CUT ==========

---- Original Message ----
From: Lee Essen lee.essen at nowonline.co.uk
Date: Thu Apr 3 12:48:05 CEST 2008

Hi,

Apologies if this is directed in the wrong place, as I suspect this is more of a kernel/USB issue than a DVB driver issue, but it does have an impact on people wanting to use this device with an NSLU2 (and I suspect it will also be a problem with many other devices.)

I have been experimenting using a variety of DVB-T USB devices with an NSLU2 with my ultimate aim being to build in a dual DVB-T device into the case and use it in very much the same way as the HDHomeRun device.

Using a DTT200U based device everything worked perfectly.

Then I started playing with a DIB0700 based device (actually an Elgato Eye-TV Diversity) and the system would just hang whenever I started using dvbstream, I got slightly different behaviour if I tried to tune it to an invalid frequency and eventually managed to get to a state when I could interrupt it before it completely locked up.

It seems that the driver was flagging an issue in the ARM architecture around not calling dma_free_coherent() with IRQ's disabled, apparently a warning was recently added to the ARM kernel so it logs a message and a stack trace each time ... this seemed to be happening so frequently it effectively locked the system up.

I did a little digging, but I'm not a kernel expert at all, and it seems that the ehci_hcd module is actually where the call is originating rather than the DVB driver itself, so I suspect that this will actually effect a significant number of the drivers when used on an ARM platform.

For the purposes of testing I removed the warning (from arch/arm/mm/ consistent line 363) and everything is fine, the driver operates perfectly and I can stream video nicely. BUT - clearly there is some kind of issue here that needs to be resolved.

More information is available at the link below, and also I have read comments suggesting that the issue has been discussed on the arm-kernel mailing list but that no resolution has yet been found.

http://forum.soft32.com/linux/ehci_hcd-map_single-unable-map-unsafe-buffer-standard-NSLU2-ftopict461241.html

For reference I'm using 2.6.24 and have tried the most recent drivers from linuxtv.org as well as a variety of others -- all seem to have the same problem (which is expected if the problem is actually in the USB system.)

Hope this is useful,

Lee.

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Hello,
I put Lee in CC.

Well, I experienced the same issue with the Terratec Cinergy Hybrid XS.
See the problem on the em28xx ML:
http://www.spinics.net/lists/linux-usb-devel/msg09511.html
This problem can occur generally on USB devices on the ixp4xx arch.
I tried to patch the linux kernel for the dma part of ixp4xx:
http://www.spinics.net/lists/arm-kernel/msg43509.html
With this patch the driver was able to tune channels instead of the famous BUG message. But I could not stream video unfortunately.
So this patch did not have any following although I expected someone to propose something better. Your solution is to decrease the buffer size and I suspect other USB drivers made a similar correction.

Cheers,
Thierry

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