Re: [PATCH] swiotlb: Fix unexpected swiotlb_alloc_coherent() failures

[Robin Murphy]

On 10/04/18 18:07, Takashi Iwai wrote:
> On Tue, 10 Apr 2018 19:06:15 +0200,
> Christoph Hellwig wrote:
> > On Tue, Apr 10, 2018 at 07:05:13PM +0200, Takashi Iwai wrote:
> > > The code refactoring by commit 0176adb00406 ("swiotlb: refactor
> > > coherent buffer allocation") made swiotlb_alloc_buffer() almost always
> > > failing due to a thinko: namely, the function evaluates the
> > > dma_coherent_ok() call incorrectly and dealing as if it's invalid.
> > > This ends up with weird errors like iwlwifi probe failure or amdgpu
> > > screen flickering.
> > >
> > > This patch corrects the logic error.
> >
> > This looks ok, although even hitting this code is a bad sign.  Do you
> > know why the previous dma_direct_alloc didn't succeed?
>
> That I have no idea, sorry.  I just figured out the regression just
> from the dmesg output of 4.16 kernel in bugzilla reports.
> Could you take a look at the bugzilla reports there?

In the first one, the machine appears to have enough RAM that most of it 
is beyond the device's 36-bit DMA mask, thus it's fairly likely for the 
initial direct alloc to come back with something unsuitable.

Robin.

> thanks,
>
> Takashi
>
> > > Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1088658
> > > Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1088902
> > > Fixes: 0176adb00406 ("swiotlb: refactor coherent buffer allocation")
> > > Cc: <stable@xxxxxxxxxxxxxxx> # v4.16+
> > > Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> > > ---
> > >   lib/swiotlb.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/lib/swiotlb.c b/lib/swiotlb.c
> > > index 47aeb04c1997..de7cc540450f 100644
> > > --- a/lib/swiotlb.c
> > > +++ b/lib/swiotlb.c
> > > @@ -719,7 +719,7 @@ swiotlb_alloc_buffer(struct device *dev, size_t size, dma_addr_t *dma_handle,
> > >   		goto out_warn;
> > >   
> > >   	*dma_handle = __phys_to_dma(dev, phys_addr);
> > > -	if (dma_coherent_ok(dev, *dma_handle, size))
> > > +	if (!dma_coherent_ok(dev, *dma_handle, size))
> > >   		goto out_unmap;
> > >   
> > >   	memset(phys_to_virt(phys_addr), 0, size);
> > > --
> > > 2.16.3
> > ---end quoted text---
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