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

[Fabien DESSENNE]

On 14/05/18 12:39, Mauro Carvalho Chehab wrote:
> 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));
>>

That's clearer now, thank you for the clarification
I am about to send patches for the sti drivers (set the DMA bit mask)

BR,
Fabien

>>> 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