On Wed, Mar 21, 2012 at 10:46:14AM -0700, Rebecca Schultz Zavin wrote: > I want to make sure I understand how this would work. I've been planning > on making cache maintenance implicit, and most of the corresponding > userspace components I've seen for android expect to do implicit cache > maintenance on these buffers if they need cached mappings. The android > framework has a logical place for this maintenance to take place. I assume > that you'd detect a buffer leaving the cpu domain by using the > dma_data_direction passed to dma_buf_map_attachment? We're definitely > pushing a bunch of complexity into the exporter, that at least on android > could easily be covered by an explicit api. I'm not dead set against it, I > just want to make sure I get it right if we go down this road. Hm, you're talking about implicit cache management, which I read as: The kernel does some magic so that userspace doesn't have to care. But I guess you mean what I'd call explicit cache management, where userspace tells the kernel which ranges to invalidate/flush? The idea is that the exporter would invalidate cpu caches at fault time. And when a dma-device wants to read from it (using the direction argument of dma_buf_map_attachement) it would shoot down the mappings, flush caches and then allow the dma op to happen. Note that this is obvously only required if the mapping is not coherent (uc/wc on arm). I agree that for cached mappings this will suck, but to get dma-buf of the ground I prefer a simpler interface at the beginning, which could get extended later on. The issue is that I expect that a few importers simply can't hanlde cache management with explicit flush/invalidate ioctl calls from userspace and we hence need coherently-looking mappings anyway. Imo the best way to enable cached mappings is to later on extend dma-buf (as soon as we have some actual exporters/importers in the mainline kernel) with an optional cached_mmap interface which requires explict prepare_mmap_access/finish_mmap_acces calls. Then if both exporter and importer support this, it could get used - otherwise the dma-buf layer could transparently fall back to coherent mappings. Cheers, Daniel -- Daniel Vetter Mail: daniel@xxxxxxxx Mobile: +41 (0)79 365 57 48 -- 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
