Hi Hugh, I've spent the last few days "playing" with get_user_pages() and mlock() and got some interesting results. It turned out that cache coherency comes into play at some point, making the overall problem more complex. Here's my current setup: - OMAP processor, based on an ARMv7 core - MMU and IOMMU - VIPT non-aliasing data cache - video capture driver that transfers data to memory using DMA - video capture application that pass userspace pointers to video buffers to the driver My goal is to make sure that, upon DMA completion, the correct data will be available to the userspace application. The first problem was to pin pages to memory, to make sure they will not be freed when the DMA is in progress. videobug-dma-sg uses get_user_pages() for that, and you nicely explained to me why this is enough. The second problem is to ensure cache coherency. As the userspace application will read data from the video buffers, those buffers will end up being cached in the processor's data cache. The driver does need to invalidate the cache before starting the DMA operation (userspace could in theory write to the buffers, but the data will be overwritten by DMA anyway, so there's no need to clean the cache). As the cache is of the VIPT (Virtual Index Physical Tag) type, cache invalidation can either be done globally (in which case the cache is flushed instead of being invalidated) or based on virtual addresses. In the last case the processor will need to look physical addresses up, either in the TLB or through hardware table walk. I can see three solutions to the DMA/cache problem. 1. Flushing the whole data cache right before starting the DMA transfer. There's no API for that in the ARM architecture, so a whole I+D cache is required. This is quite costly, we're talking about around 30 flushes per second, but it doesn't involve the MMU. That's the solution that I currently use. 2. Invalidating only the cache lines that store video buffer data. This requires a TLB lookup or a hardware table walk, so the userspace application MM context needs to be available (no problem there as where's flushing in userspace context) and all pages need to be mapped properly. This can be a problem as, as you pointed out, pages can still be unmapped from the userspace context after get_user_pages() returns. I have experienced one oops due to a kernel paging request failure: Unable to handle kernel paging request at virtual address 44e12000 pgd = c8698000 [44e12000] *pgd=8a4fd031, *pte=8cfda1cd, *ppte=00000000 Internal error: Oops: 817 [#1] PREEMPT PC is at v7_dma_inv_range+0x2c/0x44 Fixing this requires more investigation, and I'm not sure how to proceed to find out if the page fault is really caused by pages being unmapped from the userspace context. Help would be appreciated. 3. Mark the pages as non-cacheable. Depending on how the buffers are then used by userspace, the additional cache misses might destroy any benefit I would get from not flushing the cache before DMA. I'm not sure how to mark a bunch of pages as non-cacheable though. What usually happens is that video drivers allocate DMA-coherent memory themselves, but in this case I need to deal with an arbitrary buffer allocated by userspace. If someone has any experience with this, it would be appreciated. Regards, Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
