On Fri 10 Dec 2010 at 22:03:52 Janusz Krzysztofik wrote:
> Friday 10 December 2010 18:03:56 Russell King - ARM Linux napisaÅ(a):
> > On Fri, Dec 10, 2010 at 12:03:07PM +0100, Janusz Krzysztofik wrote:
> > > void __init omap1_camera_init(void *info)
> > > {
> > >
> > > struct platform_device *dev = &omap1_camera_device;
> > >
> > > + dma_addr_t paddr = omap1_camera_phys_mempool_base;
> > > + dma_addr_t size = omap1_camera_phys_mempool_size;
> > >
> > > int ret;
> > >
> > > dev->dev.platform_data = info;
> > >
> > > + if (paddr) {
> > > + if (dma_declare_coherent_memory(&dev->dev, paddr, paddr, size,
> > > + DMA_MEMORY_MAP | DMA_MEMORY_EXCLUSIVE))
> >
> > Although this works, you're ending up with SDRAM being mapped via
> > ioremap, which uses MT_DEVICE - so what is SDRAM ends up being
> > mapped as if it were a device.
> >
> > For OMAP1, which is ARMv5 or lower, device memory becomes
> > 'uncacheable, unbufferable' which is luckily what is used for the
> > DMA coherent memory on those platforms - so no technical problem
> > here.
> >
> > However, on ARMv6 and later, ioremapped memory is device memory,
> > which has different ordering wrt normal memory mappings, and may
> > appear on different busses on the CPU's interface. So, this is
> > something I don't encourage as it's unclear that the hardware will
> > work.
> >
> > Essentially, dma_declare_coherent_memory() on ARM with main SDRAM
> > should be viewed as a 'it might work, it might not, and it might
> > stop working in the future' kind of interface. In other words,
> > there is no guarantee that this kind of thing will be supported in
> > the future.
>
> I was affraid of an unswer of this kind. I'm not capable of comming
> out with any better, alternative solutions. Any hints other than
> giving up with that old videobuf-contig, coherent memory based
> interface? Or can we agree on that 'luckily uncacheable,
> unbufferable, SDRAM based DMA coherent memory' solution for now?
Russell, Tony,
Sorry for getting back to this old thread, but since my previous
attempts to provide[1] or support[2] a possibly better solution to the
problem all failed on one hand, and I can see patches not very different
from mine[3] being accepted for arch/arm/mach-{mx3,imx} during this and
previous merge windows[4][5][6] on the other, is there any chance for the
'dma_declare_coherent_memory() over a memblock_alloc()->free()->remove()
obtained area' based solution being accepted for omap1_camera as well if
I resend it refreshed?
BTW, commit 6d3163ce86dd386b4f7bda80241d7fea2bc0bb1d, "mm: check if any
page in a pageblock is reserved before marking it MIGRATE_RESERVE",
almost corrected the problem for me, allowing for very stable device
operation in videobuf_dma_contig mode once all resident programs get
settled up and no unusual activity happens, but this is still not 100%
reliable, so I think that only using a kind of memory area reservered
during boot for the purpose can be considered as free of transient
-ENOMEM issues.
Thanks,
Janusz
[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2010-December/036419.html
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2010-December/036551.html
[3] http://lists.infradead.org/pipermail/linux-arm-kernel/2010-December/034643.html
[4] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;h=164f7b5237cca2701dd2943928ec8d078877a28d
[5] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;h=031e912741746e4350204bb0436590ca0e993a7d
[6] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;h=dca7c0b4293a06d1ed9387e729a4882896abccc2
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