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

[Laurent Pinchart]

Hello,

On Tuesday, 15 May 2018 10:30:28 EEST Fabien DESSENNE wrote:
> On 14/05/18 12:39, Mauro Carvalho Chehab wrote:
> > Em Mon, 14 May 2018 07:35:03 -0300 Mauro Carvalho Chehab escreveu:
> >> Em Mon, 14 May 2018 08:00:37 +0000 Fabien DESSENNE escreveu:
> >>> On 07/05/18 17:19, Mauro Carvalho Chehab wrote:
> >>>> Em Mon, 07 May 2018 16:26:08 +0300 Laurent Pinchart escreveu:
> >>>>> 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
> >>>
> >>> 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));
> 
> That's clearer now, thank you for the clarification
> I am about to send patches for the sti drivers (set the DMA bit mask)

Some drivers call vb2_dma_contig_set_max_seg_size() and some call 
dma_coerce_mask_and_coherent(). Both are likely needed, the former telling the 
DMA mapping API about the maximum size of a scatter-gather chunk that the 
device supports (when using vb2-dma-contig that size should really be the full 
address space supported by the device as we want DMA-contiguous buffers), and 
the latter telling the DMA mapping API about the address space that is 
accessible through DMA (and thus in which address range buffers must be 
placed).

I wonder why the omap3isp driver works without a 
vb2_dma_contig_set_max_seg_size() call. Sakari, any insight ?

> >>>>>> 	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,

Laurent Pinchart