Re: [PATCH] media: vb2: unify calling of set_page_dirty_lock

[Nicolas Dufresne <nicolas@ndufresne.ca>]

Le mardi 13 mars 2018 à 20:44 -0400, Nicolas Dufresne a écrit :
> Le mercredi 18 octobre 2017 à 11:34 +0300, Stanimir Varbanov a écrit
> :
> > 
> > On 10/17/2017 05:19 PM, Nicolas Dufresne wrote:
> > > Le mardi 17 octobre 2017 à 13:14 +0300, Sakari Ailus a écrit :
> > > > On Sun, Oct 15, 2017 at 07:09:24PM -0400, Nicolas Dufresne
> > > > wrote:
> > > > > Le dimanche 15 octobre 2017 à 23:40 +0300, Sakari Ailus a
> > > > > écrit
> > > > > :
> > > > > > Hi Nicolas,
> > > > > > 
> > > > > > On Tue, Oct 10, 2017 at 11:40:10AM -0400, Nicolas Dufresne
> > > > > > wrote:
> > > > > > > Le mardi 29 août 2017 à 14:26 +0300, Stanimir Varbanov a
> > > > > > > écrit :
> > > > > > > > Currently videobuf2-dma-sg checks for dma direction for
> > > > > > > > every single page and videobuf2-dc lacks any dma
> > > > > > > > direction
> > > > > > > > checks and calls set_page_dirty_lock unconditionally.
> > > > > > > > 
> > > > > > > > Thus unify and align the invocations of
> > > > > > > > set_page_dirty_lock
> > > > > > > > for videobuf2-dc, videobuf2-sg  memory allocators with
> > > > > > > > videobuf2-vmalloc, i.e. the pattern used in vmalloc has
> > > > > > > > been
> > > > > > > > copied to dc and dma-sg.
> > > > > > > 
> > > > > > > Just before we go too far in "doing like vmalloc", I
> > > > > > > would
> > > > > > > like to
> > > > > > > share this small video that display coherency issues when
> > > > > > > rendering
> > > > > > > vmalloc backed DMABuf over various KMS/DRM driver. I can
> > > > > > > reproduce
> > > > > > > this
> > > > > > > easily with Intel and MSM display drivers using UVC or
> > > > > > > Vivid as
> > > > > > > source.
> > > > > > > 
> > > > > > > The following is an HDMI capture of the following
> > > > > > > GStreamer
> > > > > > > pipeline
> > > > > > > running on Dragonboard 410c.
> > > > > > > 
> > > > > > >     gst-launch-1.0 -v v4l2src device=/dev/video2 !
> > > > > > > video/x-
> > > > > > > raw,format=NV16,width=1280,height=720 ! kmssink
> > > > > > >     https://people.collabora.com/~nicolas/vmalloc-issue.m
> > > > > > > ov
> > > > > > > 
> > > > > > > Feedback on this issue would be more then welcome. It's
> > > > > > > not
> > > > > > > clear
> > > > > > > to me
> > > > > > > who's bug is this (v4l2, drm or iommu). The software is
> > > > > > > unlikely to
> > > > > > > be
> > > > > > > blamed as this same pipeline works fine with non-vmalloc
> > > > > > > based
> > > > > > > sources.
> > > > > > 
> > > > > > Could you elaborate this a little bit more? Which Intel CPU
> > > > > > do you
> > > > > > have
> > > > > > there?
> > > > > 
> > > > > I have tested with Skylake and Ivy Bridge and on Dragonboard
> > > > > 410c
> > > > > (Qualcomm APQ8016 SoC) (same visual artefact)
> > > > 
> > > > I presume kmssink draws on the display. Which GPU did you use?
> > > 
> > > In order, GPU will be Iris Pro 580, Intel® Ivybridge Mobile and
> > > an
> > > Adreno (3x ?). Why does it matter ? I'm pretty sure the GPU is
> > > not
> > > used
> > > on the DB410c for this use case.
> > 
> > Nicolas, for me this looks like a problem in v4l2. In the case of
> > vivid
> > the stats overlay (where the coherency issues are observed, and
> > most
> > probably the issue will be observed on the whole image but
> > fortunately
> > it is a static image pattern) are filled by the CPU but I cannot
> > see
> > where the cache is flushed. Also I'm wondering why .finish method
> > is
> > missing for dma-vmalloc mem_ops.
> > 
> > To be sure that the problem is in vmalloc v4l2 allocator, could you
> > change the allocator to dma-contig, there is a module param for
> > that
> > called 'allocators'.
> 
> I've looked into this again. I have hit the same issue but with CPU
> to
> DRM, using DMABuf allocated from DRM Dumb buffers. In that case,
> using
> DMA_BUF_IOCTL_SYNC fixes the issues.
> 
> This raises a lot of question around the model used in V4L2. As you
> mention, prepare/finish are missing in dma-vmalloc mem_ops. I'll give
> a
> try implementing that, it should cover my initial use case, but then
> I
> believe it will fail if my pipeline is:
> 
>   UVC -> in plane CPU modification -> DRM
> 
> Because we don't implement begin/end_cpu_access on our exported
> DMABuf.
> It should also fail for the following use case:
> 
>   UVC (importer) -> DRM
> 
> UVC driver won't call the remote dmabuf being/end_cpu_access method.
> This one is difficult because UVC driver and vivid don't seem to be
> aware of being an importer, exported or simply exporting to CPU
> (through mmap). I believe what we have now pretty much assumes the
> what
> we export as vmalloc is to be used by CPU only. Also, the usual
> direction used by prepare/finish ops won't work for drivers like
> vivid
> and UVC that write into the buffers using the cpu.
> 
> To be continued ...

While I was writing that, I was already outdated, as of now, we only
have one ops, called sync. This implements the to_cpu direction only.

> 
> Nicolas
> 
>