Messages in this thread | | | Subject | Re: [PATCH] jffs2 summary allocation | From | Josh Boyer <> | Date | Fri, 04 Apr 2008 20:11:10 -0500 |
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On Fri, 2008-04-04 at 16:58 -0700, David Brownell wrote: > On Friday 04 April 2008, Josh Boyer wrote: > > > > > ... This means specifically that you may _not_ use the > > > memory/addresses returned from vmalloc() for DMA. ... > > > > > > So I'm rather surprised to see *ANY* kernel code trying to do > > > that. That rule has been in effect for many, many years now. > > > > I don't think it was intentional. You're going through several layers > > here: > > > > JFFS2 -> mtd parts -> mtd dataflash -> atmel_spi. > > > > Typically MTD drivers aren't doing DMAs to flash and JFFS2 has no idea > > which particular chip driver is being used because it's abstracted by > > MTD. > > That's true ... although I can imagine using DMA to > avoid dcache trashing if its setup cost is low enough, > with either NAND or NOR chips. > > Still: in this context vmalloc() is wrong.
Agreed. One issue is that the summary code allocates a buffer that equals the eraseblock size of the underlying MTD device. For larger NAND chips, that may be up to 256KiB. I believe this is within the allowable kmalloc size for most architectures these days, but the summary code is 3 years old and was likely expecting a smaller limit. And there is always the question on whether finding that much contiguous memory will be an issue.
I don't see much harm with the actual patch itself, assuming larger kmallocs work as I think they should. It does make me wonder if we have other cases of vmalloc'd buffers being passed to lower drivers using DMA though.
josh
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