[EXT] Re: v4l2 allocated dma memory

Mon Apr 12 13:32:03 UTC 2021

Le lundi 12 avril 2021 à 07:32 +0000, Bing Song a écrit :
> I got below back trace from v4l2 m2m v4l2convert. Seems v4l2 core can handle
> cache clean. Suppose should skip below cache clean. And v4l2 should allocate
> cacheable buffer and manage cache flush/invalidate when gstbuffer map
> function. Is it right?
> 
>           --0.52%--gst_v4l2_buffer_pool_qbuf
>                                      __GI___ioctl
>                                      el0_sync
>                                      el0_sync_handler
>                                      el0_svc
>                                      do_el0_svc
>                                      el0_svc_common.constprop.0
>                                      __arm64_sys_ioctl
>                                      v4l2_ioctl
>                                      video_ioctl2
>                                      video_usercopy
>                                      __video_do_ioctl
>                                      v4l_qbuf
>                                      v4l2_m2m_ioctl_qbuf
>                                      v4l2_m2m_qbuf
>                                      vb2_qbuf
>                                      vb2_core_qbuf
>                                      __buf_prepare
>                                      __prepare_dmabuf
>                                      vb2_dc_map_dmabuf
>                                      dma_buf_map_attachment
>                                      dma_heap_map_dma_buf
>                                      dma_map_sg_attrs
>                                      dma_direct_map_sg
>                                      __pi___clean_dcache_area_poc

I thought you were using the contiguous allocator in VB2, that looks like the
SQG allocator, which is meant to be used with HW that has limited scatter list
capability (no mmu, but can deal with scattered pages). Now, if you see cache
operation happening, you likely have cache-able memory.

I'm not an expert, but I believe that VB2 have always been doing cache
invalidation on QBUF(OUTPUT). With proper information, it could indeed skip this
invalidation when the cache-able buffer was passed from another HW without being
written by the CPU.

Note you didn't share your pipeline, as zero-copy is not automatically enabled,
you may be in the copy path, in which case gst will memcopy the frame, and the
cache invalidation becomes strictly needed.

> 
> Regards,
> Bing
> 
> -----Original Message-----
> From: gstreamer-devel <gstreamer-devel-bounces at lists.freedesktop.org> On Behalf Of Nicolas Dufresne
> Sent: 2021年4月2日 20:56
> To: Discussion of the development of and with GStreamer <gstreamer-devel at lists.freedesktop.org>
> Subject: Re: [EXT] Re: v4l2 allocated dma memory
> 
> Caution: EXT Email
> 
> Le vendredi 02 avril 2021 à 01:41 +0000, Bing Song a écrit :
> > But dmabuf map/unmap will handle cache flush/invalidate. So v4l2 without an IOMMU can use cacheable buffer in gstreamer. Right?
> 
> Cache invalidation is much more complex then this, and this is very partially implemented in V4L2. So without a system wide coherent memory (without all HW component supporting CPU and device caches on the fly), using un-cacheable memory is the only way to go in this short time. This indeed can be improved and eventually fixed, but discussing this within GStreamer project won't help with that.
> 
> regards,
> Nicolas
> 
> p.s. Use linux-media mailing list or #v4l2 Freenode channel.
> 
> 
> > 
> > Regards,
> > Bing
> > 
> > -----Original Message-----
> > From: gstreamer-devel <gstreamer-devel-bounces at lists.freedesktop.org> 
> > On Behalf Of Nicolas Dufresne
> > Sent: 2021年4月2日 2:16
> > To: Discussion of the development of and with GStreamer 
> > <gstreamer-devel at lists.freedesktop.org>
> > Subject: [EXT] Re: v4l2 allocated dma memory
> > 
> > Caution: EXT Email
> > 
> > Le jeudi 01 avril 2021 à 08:17 +0000, Bing Song a écrit :
> > > Hi,
> > > 
> > > Is v4l2 video decoder output dma memory cacheable or non-cacheable? 
> > > I found videoconvert performance is bad with v4l2 video dma buffer 
> > > output buffer. Is it possible to use cacheable v4l2 dma memory?
> > 
> > It's driver decision and the kernel does not provide this information. In general, drivers without an IOMMU (like most of NXP drivers) will use non-cache- able memory allocation which is near unusable by videoconvert (often slower software decode). Most SoC have some converter capability, which can be exposed through an m2m driver and uses with v4l2convert element.
> > 
> > > 
> > > Regards,
> > > Bing
> > > _______________________________________________
> > > gstreamer-devel mailing list
> > > gstreamer-devel at lists.freedesktop.org
> > > https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fli
> > > st
> > > s.freedesktop.org%2Fmailman%2Flistinfo%2Fgstreamer-devel&data=04
> > > %7 
> > > C01%7Cbing.song%40nxp.com%7C5ff6c3d4fb7640cbb09a08d8f53a3b07%7C686ea
> > > 1d 
> > > 3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C637528977733606983%7CUnknown%7CT
> > > WF 
> > > pbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI
> > > 6M
> > > n0%3D%7C1000&sdata=V6a2V6x1XfGr6wEosiXoBC8eA85YjpwvsE%2Fs04fD79k
> > > %3
> > > D&reserved=0
> > 
> > 
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> > 3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C637529649756904640%7CUnknown%7CTWF
> > pbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6M
> > n0%3D%7C1000&sdata=tedEcL98YrPp6kwmPRqTZF6CrMAHdlGRS4fEluPqjH0%3D&
> > amp;reserved=0 _______________________________________________
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> > C01%7Cbing.song%40nxp.com%7C3fbc38a316e04f6c5e0308d8f5d6b2ba%7C686ea1d
> > 3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C637529649756904640%7CUnknown%7CTWF
> > pbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6M
> > n0%3D%7C1000&sdata=tedEcL98YrPp6kwmPRqTZF6CrMAHdlGRS4fEluPqjH0%3D&
> > amp;reserved=0
> 
> 
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