Tomasz Figa
2020-Aug-20 17:33 UTC
[Nouveau] [PATCH 05/28] media/v4l2: remove V4L2-FLAG-MEMORY-NON-CONSISTENT
On Thu, Aug 20, 2020 at 6:54 PM Christoph Hellwig <hch at lst.de> wrote:> > On Thu, Aug 20, 2020 at 12:05:29PM +0200, Tomasz Figa wrote: > > The UAPI and V4L2/videobuf2 changes are in good shape and the only > > wrong part is the use of DMA API, which was based on an earlier email > > guidance anyway, and a change to the synchronization part . I find > > conclusions like the above insulting for people who put many hours > > into designing and implementing the related functionality, given the > > complexity of the videobuf2 framework and how ill-defined the DMA API > > was, and would feel better if such could be avoided in future > > communication. > > It wasn't meant to be too insulting, but I found this out when trying > to figure out how to just disable it. But it also ends up using > the actual dma attr flags for it's own consistency checks, so just > not setting the flag did not turn out to work that easily. >Yes, sadly the videobuf2 ended up becoming quite counterintuitive after growing for the long years and that is reflected in the design of this feature as well. I think we need to do something about it.> But in general it helps to add a few more people to the Cc list for > such things that do stranger things. Especially if you think you did > it based on the advice of those people.Indeed, we should have CCed you and other DMA folks. Sergey who worked on this series is quite new to these areas of the kernel (although not to the kernel itself) and it's my fault for not explicitly letting him know to do that. Best regards, Tomasz
Christoph Hellwig
2020-Sep-01 11:06 UTC
[Nouveau] [PATCH 05/28] media/v4l2: remove V4L2-FLAG-MEMORY-NON-CONSISTENT
On Thu, Aug 20, 2020 at 07:33:48PM +0200, Tomasz Figa wrote:> > It wasn't meant to be too insulting, but I found this out when trying > > to figure out how to just disable it. But it also ends up using > > the actual dma attr flags for it's own consistency checks, so just > > not setting the flag did not turn out to work that easily. > > > > Yes, sadly the videobuf2 ended up becoming quite counterintuitive > after growing for the long years and that is reflected in the design > of this feature as well. I think we need to do something about it.So I'm about to respin the series and wonder how we should proceed. I've failed to come up with a clean patch to keep the flag and make it a no-op. Can you or your team give it a spin? Also I wonder if the flag should be renamed from NON_CONSISTENT to NON_COHERENT - the consistent thing is a weird wart from the times the old PCI DMA API that is mostly gone now.
Tomasz Figa
2020-Sep-01 15:02 UTC
[Nouveau] [PATCH 05/28] media/v4l2: remove V4L2-FLAG-MEMORY-NON-CONSISTENT
On Tue, Sep 1, 2020 at 1:06 PM Christoph Hellwig <hch at lst.de> wrote:> > On Thu, Aug 20, 2020 at 07:33:48PM +0200, Tomasz Figa wrote: > > > It wasn't meant to be too insulting, but I found this out when trying > > > to figure out how to just disable it. But it also ends up using > > > the actual dma attr flags for it's own consistency checks, so just > > > not setting the flag did not turn out to work that easily. > > > > > > > Yes, sadly the videobuf2 ended up becoming quite counterintuitive > > after growing for the long years and that is reflected in the design > > of this feature as well. I think we need to do something about it. > > So I'm about to respin the series and wonder how we should proceed. > I've failed to come up with a clean patch to keep the flag and make > it a no-op. Can you or your team give it a spin? >Okay, I'll take a look.> Also I wonder if the flag should be renamed from NON_CONSISTENT > to NON_COHERENT - the consistent thing is a weird wart from the times > the old PCI DMA API that is mostly gone now.It originated from the DMA_ATTR_NON_CONSISTENT flag, but agreed that NON_COHERENT would be more consistent (pun not intended) with the rest of the DMA API given the removal of that flag. Let me see if we can still change it. Best regards, Tomasz
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