Re: [Linaro-mm-sig] [PATCH 2/3] dma-buf: add support for kernel cpu access
On Fri, Mar 2, 2012 at 23:53, Rob Clark rob.cl...@linaro.org wrote: nitially the expectation was that userspace would not pass a buffer to multiple subsystems for writing (or that if it did, it would get the undefined results that one could expect).. so dealing w/ synchronization was punted. Imo synchronization should not be part of the dma_buf core, i.e. userspace needs to ensure that access is synchronized. begin/end_cpu_access are the coherency brackets (like map/unmap for device dma). And if userspace asks for a gun and some bullets, the kernel should just deliver. Even in drm/i915 gem land we don't (and simply can't) make any promises about concurrent reads/writes/ioctls. I expect, though, that one of the next steps is some sort of sync-object mechanism to supplement dmabuf Imo the only reason to add sync objects as explicit things is to make device-to-device sync more efficient by using hw semaphores and signalling lines. Or maybe a quick irq handler in the kernel that kicks of the next device. I don't think we should design these to make userspace simpler. Cheers, Daniel -- Daniel Vetter daniel.vet...@ffwll.ch - +41 (0) 79 365 57 48 - http://blog.ffwll.ch -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [Linaro-mm-sig] [PATCH 2/3] dma-buf: add support for kernel cpu access
On Thu, 1 Mar 2012 16:36:00 +0100, Daniel Vetter daniel.vet...@ffwll.ch wrote: Big differences to other contenders in the field (like ion) is that this also supports highmem, so we have to split up the cpu access from the kernel side into a prepare and a kmap step. Prepare is allowed to fail and should do everything required so that the kmap calls can succeed (like swapin/backing storage allocation, flushing, ...). More in-depth explanations will follow in the follow-up documentation patch. Changes in v2: - Clear up begin_cpu_access confusion noticed by Sumit Semwal. - Don't automatically fallback from the _atomic variants to the non-atomic variants. The _atomic callbacks are not allowed to sleep, so we want exporters to make this decision explicit. The function signatures are explicit, so simpler exporters can still use the same function for both. - Make the unmap functions optional. Simpler exporters with permanent mappings don't need to do anything at unmap time. Are we going to have to have a dma_buf-ops-begin_async_access(me, dir) variant for coherency control of rendering with an imported dma_buf? There is also no concurrency control here between multiple importers doing simultaneous begin_cpu_access(). I presume that is going to be a common function across all exporters so the midlayer might offer a semaphore as a library function and then the dma_buf-ops-begin_cpu_access() becomes mandatory as at a minimum it has to point to the default implementation. -Chris -- Chris Wilson, Intel Open Source Technology Centre -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [Linaro-mm-sig] [PATCH 2/3] dma-buf: add support for kernel cpu access
On Fri, Mar 2, 2012 at 4:38 PM, Chris Wilson ch...@chris-wilson.co.uk wrote: On Thu, 1 Mar 2012 16:36:00 +0100, Daniel Vetter daniel.vet...@ffwll.ch wrote: Big differences to other contenders in the field (like ion) is that this also supports highmem, so we have to split up the cpu access from the kernel side into a prepare and a kmap step. Prepare is allowed to fail and should do everything required so that the kmap calls can succeed (like swapin/backing storage allocation, flushing, ...). More in-depth explanations will follow in the follow-up documentation patch. Changes in v2: - Clear up begin_cpu_access confusion noticed by Sumit Semwal. - Don't automatically fallback from the _atomic variants to the non-atomic variants. The _atomic callbacks are not allowed to sleep, so we want exporters to make this decision explicit. The function signatures are explicit, so simpler exporters can still use the same function for both. - Make the unmap functions optional. Simpler exporters with permanent mappings don't need to do anything at unmap time. Are we going to have to have a dma_buf-ops-begin_async_access(me, dir) variant for coherency control of rendering with an imported dma_buf? There is also no concurrency control here between multiple importers doing simultaneous begin_cpu_access(). I presume that is going to be a common function across all exporters so the midlayer might offer a semaphore as a library function and then the dma_buf-ops-begin_cpu_access() becomes mandatory as at a minimum it has to point to the default implementation. Initially the expectation was that userspace would not pass a buffer to multiple subsystems for writing (or that if it did, it would get the undefined results that one could expect).. so dealing w/ synchronization was punted. I expect, though, that one of the next steps is some sort of sync-object mechanism to supplement dmabuf BR, -R -Chris -- Chris Wilson, Intel Open Source Technology Centre -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
