Messages in this thread | | | Date | Tue, 3 Nov 2009 11:00:18 +0100 | From | Johannes Stezenbach <> | Subject | Re: [PATCH] mtd: m25p80: make command buffer DMA-safe |
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On Tue, Nov 03, 2009 at 08:36:05AM +0200, Artem Bityutskiy wrote: > On Wed, 2009-10-28 at 14:21 +0100, Johannes Stezenbach wrote: > > spi_write() requires the buffer to be DMA-safe, kmalloc() > > it seperately to ensure this. > > Even though it is just 4 or 5 bytes it can do DMA? Does not sound too > sane to use DMA in that case. Does this patch fix a real error? > > I do not know much about SPI, but for me it sounds like there should be > a method to ask SPI to avoid using DMA, and you should use that method.
It fixes a real error -- with an out-of-tree driver and ancient kernel. For the flash read/write case it is used with list-DMA (e.g. write 4 bytes, read 64KB). There are extensive comments in include/linux/spi/spi.h which document the DMA-safe requirement of the buffers.
Thanks Johannes
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