Hi Laurent, Tomasz, On 06/10/2012 11:28 PM, Laurent Pinchart wrote: > Hi Tomasz, > > On Friday 08 June 2012 16:31:31 Tomasz Stanislawski wrote: >> Hi Laurent and Subash, >> >> I confirm the issue found by Subash. The function vb2_dc_kaddr_to_pages does >> fail for some occasions. The failures are rather strange like 'got 95 of >> 150 pages'. It took me some time to find the reason of the problem. >> >> I found that dma_alloc_coherent for iommu an ARM does use ioremap_page_range >> to map a buffer to the kernel space. The mapping is done by updating the >> page-table. >> >> The problem is that any process has a different first-level page-table. The >> ioremap_page_range updates only the table for init process. The PT present >> in current->mm shares a majority of entries of 1st-level PT at kernel range >> (above 0xc0000000) but *not all*. That is why vb2_dc_kaddr_to_pages worked >> for small buffers and occasionally failed for larger buffers. >> >> I found two ways to fix this problem. >> a) use&init_mm instead of current->mm while creating an artificial vma >> b) access the dma memory by calling >> *((volatile int *)kaddr) = 0; >> before calling follow_pfn >> This way a fault is generated and the PT is >> updated by copying entries from init_mm. >> >> What do you think about presented solutions? > > Just to be sure, this is a hack until dma_get_sgtable is available, and it > won't make it to mainline, right ? In that case using init_mm seem easier. Although I agree adding a hack for timebeing, why not use the dma_get_sgtable() RFC itself to solve this in clean way? The hacks anyways cannot go into mainline when vb2 patches get merged.
Regards, Subash >
