Re: Are media drivers abusing of GFP_DMA? - was: Re: [LSF/MM TOPIC NOTES] x86 ZONE_DMA love

[Mauro Carvalho Chehab]

Em Mon, 14 May 2018 07:35:03 -0300
Mauro Carvalho Chehab <mchehab+samsung@xxxxxxxxxx> escreveu:

> Hi Fabien,
> 
> Em Mon, 14 May 2018 08:00:37 +0000
> Fabien DESSENNE <fabien.dessenne@xxxxxx> escreveu:
> 
> > On 07/05/18 17:19, Mauro Carvalho Chehab wrote:
> > > Em Mon, 07 May 2018 16:26:08 +0300
> > > Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> escreveu:
> > >  
> > >> Hi Mauro,
> > >>
> > >> On Saturday, 5 May 2018 19:08:15 EEST Mauro Carvalho Chehab wrote:  
> > >>> There was a recent discussion about the use/abuse of GFP_DMA flag when
> > >>> allocating memories at LSF/MM 2018 (see Luis notes enclosed).
> > >>>
> > >>> The idea seems to be to remove it, using CMA instead. Before doing that,
> > >>> better to check if what we have on media is are valid use cases for it, or
> > >>> if it is there just due to some misunderstanding (or because it was
> > >>> copied from some other code).
> > >>>
> > >>> Hans de Goede sent us today a patch stopping abuse at gspca, and I'm
> > >>> also posting today two other patches meant to stop abuse of it on USB
> > >>> drivers. Still, there are 4 platform drivers using it:
> > >>>
> > >>> 	$ git grep -l -E "GFP_DMA\\b" drivers/media/
> > >>> 	drivers/media/platform/omap3isp/ispstat.c
> > >>> 	drivers/media/platform/sti/bdisp/bdisp-hw.c
> > >>> 	drivers/media/platform/sti/hva/hva-mem.c  
> > 
> > Hi Mauro,
> > 
> > The two STI drivers (bdisp-hw.c and hva-mem.c) are only expected to run 
> > on ARM platforms, not on x86.
> > Since this thread deals with x86 & DMA trouble, I am not sure that we 
> > actually have a problem for the sti drivers.
> > 
> > There are some other sti drivers that make use of this GFP_DMA flag 
> > (drivers/gpu/drm/sti/sti_*.c) and it does not seem to be a problem.
> > 
> > Nevertheless I can see that the media sti drivers depend on COMPILE_TEST 
> > (which is not the case for the DRM ones).
> > Would it be an acceptable solution to remove the COMPILE_TEST dependency?
> 
> This has nothing to do with either x86 or COMPILE_TEST. The thing is
> that there's a plan for removing GFP_DMA from the Kernel[1], as it was
> originally meant to be used only by old PCs, where the DMA controllers
> used only  on the bottom 16 MB memory address (24 bits). IMHO, it is 
> very unlikely that any ARM SoC have such limitation.
> 
> [1] https://lwn.net/Articles/753273/ (article will be freely available
> on May, 17)

Btw, you can also read about that at:
	https://lwn.net/Articles/753274/

> 
> Anyway, before the removal of GFP_DMA happens, I'd like to better 
> understand why we're using it at media, and if we can, instead,
> set the DMA bit mask, just like almost all other media drivers
> that require to confine DMA into a certain range do. In the case
> of ARM, this is what we currently have:
> 
> drivers/media/platform/exynos-gsc/gsc-core.c:   vb2_dma_contig_set_max_seg_size(dev, DMA_BIT_MASK(32));
> drivers/media/platform/exynos4-is/fimc-core.c:  vb2_dma_contig_set_max_seg_size(dev, DMA_BIT_MASK(32));
> drivers/media/platform/exynos4-is/fimc-is.c:    vb2_dma_contig_set_max_seg_size(dev, DMA_BIT_MASK(32));
> drivers/media/platform/exynos4-is/fimc-lite.c:  vb2_dma_contig_set_max_seg_size(dev, DMA_BIT_MASK(32));
> drivers/media/platform/mtk-mdp/mtk_mdp_core.c:  vb2_dma_contig_set_max_seg_size(&pdev->dev, DMA_BIT_MASK(32));
> drivers/media/platform/omap3isp/isp.c:  ret = dma_coerce_mask_and_coherent(isp->dev, DMA_BIT_MASK(32));
> drivers/media/platform/s5p-g2d/g2d.c:   vb2_dma_contig_set_max_seg_size(&pdev->dev, DMA_BIT_MASK(32));
> drivers/media/platform/s5p-jpeg/jpeg-core.c:    vb2_dma_contig_set_max_seg_size(&pdev->dev, DMA_BIT_MASK(32));
> drivers/media/platform/s5p-mfc/s5p_mfc.c:                                       DMA_BIT_MASK(32));
> drivers/media/platform/s5p-mfc/s5p_mfc.c:                                       DMA_BIT_MASK(32));
> drivers/media/platform/s5p-mfc/s5p_mfc.c:       vb2_dma_contig_set_max_seg_size(dev, DMA_BIT_MASK(32));
> 
> > 
> > BR
> > 
> > Fabien
> > 
> > >>> 	drivers/media/spi/cxd2880-spi.c
> > >>>
> > >>> Could you please check if GFP_DMA is really needed there, or if it is
> > >>> just because of some cut-and-paste from some other place?  
> > >> I started looking at that for the omap3isp driver but Sakari beat me at
> > >> submitting a patch. GFP_DMA isn't needed for omap3isp.
> > >>  
> > > Thank you both for looking into it.
> > >
> > > Regards,
> > > Mauro
> > >
> > >
> > >
> > > Thanks,
> > > Mauro  
> 
> 
> 
> Thanks,
> Mauro



Thanks,
Mauro