Messages in this thread | | | From | Thomas Koeller <> | Subject | DMA using data buffer vmapped in kernel space | Date | Thu, 4 Mar 2010 22:11:08 +0100 |
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Hi,
I am writing a driver that loads a firmware blob into its device. The driver calls request_firmware() and gets a struct firmware * that contains a data buffer pointer in its 'data' member. The buffer is then passed to a SPI driver to send it to the device.
The SPI driver uses DMA and, in preparation for that, eventually calls dma_cache_maint() (contained in arch/arm/mm/dma-mapping.c). At this point, the whole thing goes bad because there is a check:
BUG_ON(!virt_addr_valid(start) || !virt_addr_valid(start + size - 1));
and the address obtained via request_firmware() fails this check.
The request_firmware() function creates the data buffer by allocating a number of single pages in a loop, places them in an array, and finally creates a kernel mapping by calling vmap(). The vmap() result is the buffer address passed to the DMA.
I do not understand the purpose of the failing check. The virt_addr_valid() macro checks whether its argument is below high_memory. But why would the virtual address of a DMA data buffer matter at all? After all, the pages are resident, so I cannot see any problem here. Can anybody explain?
Thomas [unhandled content-type:application/pgp-signature] | |