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Bug#356458: [tbm@cyrius.com: Re: Bug#356458: linux-2.6: please include bigphysarea patch]

On Mon, Apr 24, 2006 at 09:09:39PM -0700, Andrew Morton wrote:
> I don't know what to suggest, really.  I don't think that adding complexity
> to MM to support rare and broken hardware is likely to be welcome.

Like I said, it's hardly rare hardware.  DC10plus is a very popular
capture card, and similar to LinuxMediaLabs LML33 which requires the
same patch (in a Linux-branded product no less).  Lack of SG-DMA is not
a new problem either:

And users are being misled and in for a surprise:
"v4l_bufsize=2048 indicates the size of the reserved bigphys buffer, it's
 enabled by default in Debian kernels (CONFIG_BIGPHYS_AREA=y), so all you
 need to do is to add to your append line in /etc/lilo.conf a
 bigphysarea parameter with the buffer size, this way the kernel will
 reserve this amount of memory as video buffer."

bigphysarea was a good enough solution for Debian to ship at one time;
unfortunately, the current Debian kernel maintainers believe the patch
should be merged upstream instead (I Cc: them on this mail), while
upstream argues with the previously happy users that the patch is badly
designed and their hardware is broken besides.  I'm mostly kidding
there, but don't you agree that this would be a frustrating situation
for a non-technical user?  I'm not arguing that bigphysarea is a good
solution, since I'm incompetent to judge its merits within the context
of the kernel design (and parts of the patch do appear dodgy at a
glance), but it is currently the only solution unless we figure out
where to go from here.

> What are the requirements here?  How much physically contig memory?  Are
> there any bus addressing limitations on it?

2 to 4MB depending on the size of the framebuffer (realistically,
720x480x24bpp should be the upper limit, 640x480x24bpp for square pixel
hardware).  I have no idea wrt your second question, but I believe that
contiguous physical memory from the frame grabber's DMA base address is
the only requirement.  The developers at mjpeg.sourceforge.net know the
hardware far better than me, since I'm just a user of that project.
Would a dialog with Ronald help you decide what to do?

Ryan Underwood, <nemesis@icequake.net>

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