Hi Russell, On Wed, Mar 29, 2017 at 11:03 AM, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> wrote: > On Wed, Mar 29, 2017 at 10:51:05AM -0600, Shuah Khan wrote: >> Hi Russell, >> >> On Wed, Mar 29, 2017 at 10:33 AM, Russell King - ARM Linux >> <linux@xxxxxxxxxxxxxxx> wrote: >> > Okay, I'm about to merge the following patch for -rc, since refusing >> > to create a scattertable for non-page backed memory is the only valid >> > solution for that case. I'm intending to queue this for -rc this >> > evening. >> >> I was bit sidetracked and getting back to this today. I am just about >> to test this change on my system. I will test your patch and send you >> results. >> >> This might not help my use-case - I suspect it will trip this check. >> That said I will send you results very soon. > > I do hope that it _does_ trip this check - the result should be that you > get an error without the kernel oopsing. Agreed. I would rather have it fail gracefully earlier than limp along and die later. I would make a suggestion based on these results. Please consider adding a dev_err() to if (!pfn_valid(pfn)) condition to indicate why get_sgtable failed including __func__ I worry about adding dev_err() to a core function, but in this case, it would be helpful to do to make it very clear that the requested use-case can't be supported. Results: [ 235.706048] platform 11000000.codec:left: failed to get scatterlist from DMA API The following is a WARN_ON from vb2_dc_get_dmabuf() - probably should go away. I will send a patch removing the WARN_ON to linux-media [ 235.711985] ------------[ cut here ]------------ [ 235.712001] WARNING: CPU: 6 PID: 1987 at drivers/media/v4l2-core/videobuf2-dma-contig.c:402 vb2_dc_get_dmabuf+0x130/0x168 [videobuf2_dma_contig] [ 235.712005] Modules linked in: cpufreq_userspace cpufreq_powersave cpufreq_conservative s5p_jpeg s5p_mfc exynos_gsc v4l2_mem2mem videobuf2_dma_contig v4l2_common videobuf2_memops videobuf2_v4l2 videobuf2_core videodev media [ 235.712063] CPU: 6 PID: 1987 Comm: qtdemux0:sink Not tainted 4.11.0-rc2-00001-g49214d0-dirty #11 [ 235.712068] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree) [ 235.712085] [<c010e244>] (unwind_backtrace) from [<c010aea0>] (show_stack+0x10/0x14) [ 235.712096] [<c010aea0>] (show_stack) from [<c033ef50>] (dump_stack+0x78/0x8c) [ 235.712106] [<c033ef50>] (dump_stack) from [<c011b3c8>] (__warn+0xe8/0x100) [ 235.712113] [<c011b3c8>] (__warn) from [<c011b490>] (warn_slowpath_null+0x20/0x28) [ 235.712122] [<c011b490>] (warn_slowpath_null) from [<bf20d8b8>] (vb2_dc_get_dmabuf+0x130/0x168 [videobuf2_dma_contig]) [ 235.712141] [<bf20d8b8>] (vb2_dc_get_dmabuf [videobuf2_dma_contig]) from [<bf151ec4>] (vb2_core_expbuf+0x1d4/0x2d8 [videobuf2_core]) [ 235.712159] [<bf151ec4>] (vb2_core_expbuf [videobuf2_core]) from [<bf19c198>] (vb2_expbuf+0x2c/0x34 [videobuf2_v4l2]) [ 235.712195] [<bf19c198>] (vb2_expbuf [videobuf2_v4l2]) from [<bf04b834>] (__video_do_ioctl+0x204/0x2fc [videodev]) [ 235.712241] [<bf04b834>] (__video_do_ioctl [videodev]) from [<bf04b390>] (video_usercopy+0x24c/0x4e0 [videodev]) [ 235.712285] [<bf04b390>] (video_usercopy [videodev]) from [<bf0465a4>] (v4l2_ioctl+0xa0/0xd8 [videodev]) [ 235.712316] [<bf0465a4>] (v4l2_ioctl [videodev]) from [<c01fe920>] (do_vfs_ioctl+0x9c/0x8e0) [ 235.712325] [<c01fe920>] (do_vfs_ioctl) from [<c01ff198>] (SyS_ioctl+0x34/0x5c) [ 235.712334] [<c01ff198>] (SyS_ioctl) from [<c0107740>] (ret_fast_syscall+0x0/0x3c) [ 235.712339] ---[ end trace cdfda037de46497c ]--- > >> I have been thinking about this some. I would like to see if we can >> provide a way forward to address the cases where coherent memory is >> not guaranteed to be backed by struct pages. We know the memory isn't >> backed at alloc time in dma_alloc_coherent(). Can we key off of that >> maybe add a new attr flag to avoid page lookups. I am willing to work >> on the fixing it. > > What we need is to revise the dma-buf API and DMA APIs to allow passing > coherent memory through it - there is no current provision in the DMA APIs > to have coherent memory mapped for two peripherals. I will explore adding the support. I do think there is value in being able to use coherent memory buffers for DMA. thanks, -- Shuah > > -- > RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ > FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up > according to speedtest.net.