On Tue, Aug 28, 2018 at 03:25:55PM +0200, Linus Walleij wrote: > On Tue, Aug 28, 2018 at 11:21 AM Christoph Hellwig <hch@xxxxxx> wrote: > > > > + dev->dev.coherent_dma_mask = DMA_BIT_MASK(32); > > > + if (!dev->dev.dma_mask) > > > + dev->dev.dma_mask = &dev->dev.coherent_dma_mask; > > > > We should never set dma_mask to point to the coherent_dma_mask, > > as that will cause problems with devices that have different > > mask for the two. > > > > How about something like this? > (...) > > + u64 dma_mask; > > We did that before, so this becomes effectively a revert of: > commit 446b2a9380b64b9d7410d86ee8226031e03645cf > > DMA-API: amba: get rid of separate dma_mask > > AMBA Primecell devices always treat streaming and coherent DMA exactly > the same, so there's no point in having the masks separated. > > So there is some history around this. > > There is also some code in drivers/amba/bus.c affected by that > and I need to look over all static amba device allocations if we > reintroduce this. I don't have the rest of this thread to read... But yes, the fundamental fact is that AMBA devices don't have any care about the differences between coherent and streaming DMA. The distinction that we make in the kernel is purely a software one when it comes to these devices. Most AMBA devices themselves are not DMA capable, as they are only connected to the APB (Amba peripheral bus) and they rely on a separate DMA engine for their DMA. APB devices should not have DMA masks - their DMA capabilities are entirely down to the DMA controller. So, the majority of AMBA devices should not have any DMA masks. Only those connected to a bus that they can master on (eg AXI) should have DMA masks - things like the PL08x DMA controllers, PL11x LCD controllers, etc. As I've said above, there is no difference between streaming and coherent DMA for these devices. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 13.8Mbps down 630kbps up According to speedtest.net: 13Mbps down 490kbps up _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
