linux-kernel - Re: [PATCH] media: uvcvideo: Use streaming DMA APIs to transfer buffers

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Date:   Mon, 30 Sep 2019 01:23:10 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Nicolas Boichat <drinkcat@...omium.org>
Cc:     Shik Chen <shik@...omium.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Tomasz Figa <tfiga@...omium.org>, notify@...nel.org,
        Keiichi Watanabe <keiichiw@...omium.org>,
        Ricky Liang <jcliang@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        lkml <linux-kernel@...r.kernel.org>,
        kieran.bingham@...asonboard.com, Vlastimil Babka <vbabka@...e.cz>,
        Christoph Lameter <cl@...ux.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] media: uvcvideo: Use streaming DMA APIs to transfer
 buffers

On Sat, Sep 28, 2019 at 11:33:16AM +0800, Nicolas Boichat wrote:
> > +static gfp_t uvc_alloc_gfp_flags(struct device *dev)
> > +{
> > +       u64 mask = dma_get_mask(dev);
> > +
> > +       if (dev->bus_dma_mask)
> > +               mask &= dev->bus_dma_mask;
> > +
> > +       if (mask < DMA_BIT_MASK(32) && IS_ENABLED(CONFIG_ZONE_DMA))
> > +               return GFP_DMA;
> > +
> > +       if (mask < DMA_BIT_MASK(64)) {
> > +               if (IS_ENABLED(CONFIG_ZONE_DMA32))
> > +                       return GFP_DMA32;
> 
> We're hitting issues with this on 64-bit ARM platform, where
> ZONE_DMA32 is enabled (default), the kmalloc allocation with GFP_DMA32
> fails.
> 
> There is no slab cache for GFP_DMA32, so those calls are expected to
> fail, AFAICT there are no other (working) kmalloc(..., .. | GFP_DMA32)
> users in the kernel, so I don't think we want to add a cache. If this
> helps, some discussion here why the cache wasn't added:
> https://lore.kernel.org/patchwork/patch/1009563/#1198622

And drivers really have no business looking at the dma mask.  I have
a plan for dma_alloc_pages API that could replace that cruft, but
until then please use GFP_KERNEL and let the dma subsystem bounce
buffer if needed.