Re: [PATCH] drm: add func to better detect wether swiotlb is needed

[Konrad Rzeszutek Wilk]

.snip..
> > -u64 drm_get_max_iomem(void)
> > +bool drm_need_swiotlb(int dma_bits)
> >  {
> > struct resource *tmp;
> > resource_size_t max_iomem = 0;
> > 
> > +   /*
> > +* Xen paravirtual hosts require swiotlb regardless of requested dma
> > +* transfer size.
> > +*
> > +* NOTE: Really, what it requires is use of the dma_alloc_coherent
> > +*   allocator used in ttm_dma_populate() instead of
> > +*   ttm_populate_and_map_pages(), which bounce buffers so much
> > in
> > +*   Xen it leads to swiotlb buffer exhaustion.
> > +*/
> > +   if (xen_pv_domain())
> 
> I've not been following all of the ins and outs of the discussion on this so 
> apologies if I'm missing some context, but...
> 
> This looks like the wrong test to me. I think it should be:
> 
> if ( xen_swiotlb )

Ah, that could be as well. 
> 
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Re: amdgpu/TTM oopses since merging swiotlb_dma_ops into the dma_direct code

[Konrad Rzeszutek Wilk]

On Thu, Jan 10, 2019 at 04:26:43PM +0100, Sibren Vasse wrote:
> On Thu, 10 Jan 2019 at 14:57, Christoph Hellwig  wrote:
> >
> > On Thu, Jan 10, 2019 at 10:59:02AM +0100, Michel Dänzer wrote:
> > >
> > > Hi Christoph,
> > >
> > >
> > > https://bugs.freedesktop.org/109234 (please ignore comments #6-#9) was
> > > bisected to your commit 55897af63091 "dma-direct: merge swiotlb_dma_ops
> > > into the dma_direct code". Any ideas?
> >
> > From the trace it looks like we git the case where swiotlb tries
> > to copy back data from a bounce buffer, but hits a dangling or NULL
> > pointer.  So a couple questions for the submitter:
> My apologies if I misunderstand something, this subject matter is new to me.
> 
> >
> >  - does the system have more than 4GB memory and thus use swiotlb?
> My system has 8GB memory. The other report on the bug tracker had 16GB.
> 
> >(check /proc/meminfo, and if something SWIOTLB appears in dmesg)
> /proc/meminfo: https://ptpb.pw/4rxI
> Can I grep dmesg for a string?

Can you attach the 'dmesg'? 

> 
> >  - does the device this happens on have a DMA mask smaller than
> >the available memory, that is should swiotlb be used here to start
> >with?
> It's a MSI Radeon RX 570 Gaming X 4GB. The other report was a RX 580.
> lshw output: https://ptpb.pw/6s0H
> 
> 
> Regards,
> 
> Sibren
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