On 1/21/19 2:20 PM, Liam Mark wrote: > On Mon, 21 Jan 2019, Andrew F. Davis wrote: > >> On 1/21/19 1:44 PM, Liam Mark wrote: >>> On Mon, 21 Jan 2019, Christoph Hellwig wrote: >>> >>>> On Sat, Jan 19, 2019 at 08:50:41AM -0800, Laura Abbott wrote: >>>>>> And who is going to decide which ones to pass? And who documents >>>>>> which ones are safe? >>>>>> >>>>>> I'd much rather have explicit, well documented dma-buf flags that >>>>>> might get translated to the DMA API flags, which are not error checked, >>>>>> not very well documented and way to easy to get wrong. >>>>>> >>>>> >>>>> I'm not sure having flags in dma-buf really solves anything >>>>> given drivers can use the attributes directly with dma_map >>>>> anyway, which is what we're looking to do. The intention >>>>> is for the driver creating the dma_buf attachment to have >>>>> the knowledge of which flags to use. >>>> >>>> Well, there are very few flags that you can simply use for all calls of >>>> dma_map*. And given how badly these flags are defined I just don't want >>>> people to add more places where they indirectly use these flags, as >>>> it will be more than enough work to clean up the current mess. >>>> >>>> What flag(s) do you want to pass this way, btw? Maybe that is where >>>> the problem is. >>>> >>> >>> The main use case is for allowing clients to pass in >>> DMA_ATTR_SKIP_CPU_SYNC in order to skip the default cache maintenance >>> which happens in dma_buf_map_attachment and dma_buf_unmap_attachment. In >>> ION the buffers aren't usually accessed from the CPU so this allows >>> clients to often avoid doing unnecessary cache maintenance. >>> >> >> How can a client know that no CPU access has occurred that needs to be >> flushed out? >> > > I have left this to clients, but if they own the buffer they can have the > knowledge as to whether CPU access is needed in that use case (example for > post-processing). > > For example with the previous version of ION we left all decisions of > whether cache maintenance was required up to the client, they would use > the ION cache maintenance IOCTL to force cache maintenance only when it > was required. > In these cases almost all of the access was being done by the device and > in the rare cases CPU access was required clients would initiate the > required cache maintenance before and after the CPU access. > I think we have different definitions of "client", I'm talking about the DMA-BUF client (the importer), that is who can set this flag. It seems you mean the userspace application, which has no control over this flag. > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, > a Linux Foundation Collaborative Project > _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
