Re: [PATCH] dma-pool: Do not allocate pool memory from CMA

[Nicolas Saenz Julienne]

On Tue, 2020-07-28 at 11:13 +0200, Christoph Hellwig wrote:
> On Mon, Jul 27, 2020 at 07:56:56PM +0200, Nicolas Saenz Julienne wrote:
> > Hi Christoph,
> > thanks for having a look at this!
> > 
> > On Fri, 2020-07-24 at 15:41 +0200, Christoph Hellwig wrote:
> > > Yes, the iommu is an interesting case, and the current code is
> > > wrong for that.
> > 
> > Care to expand on this? I do get that checking dma_coherent_ok() on memory
> > that'll later on be mapped into an iommu is kind of silly, although I think
> > harmless in Amir's specific case, since devices have wide enough dma-
ranges. 
> > Is
> > there more to it?
> 
> I think the problem is that it can lead to not finding suitable memory.
> 
> > > Can you try the patch below?  It contains a modified version of Nicolas'
> > > patch to try CMA again for the expansion and a new (for now hackish) way
> > > to
> > > not apply the addressability check for dma-iommu allocations.
> > > 
> > > diff --git a/kernel/dma/pool.c b/kernel/dma/pool.c
> > > index 6bc74a2d51273e..ec5e525d2b9309 100644
> > > --- a/kernel/dma/pool.c
> > > +++ b/kernel/dma/pool.c
> > > @@ -3,7 +3,9 @@
> > >   * Copyright (C) 2012 ARM Ltd.
> > >   * Copyright (C) 2020 Google LLC
> > >   */
> > > +#include <linux/cma.h>
> > >  #include <linux/debugfs.h>
> > > +#include <linux/dma-contiguous.h>
> > >  #include <linux/dma-direct.h>
> > >  #include <linux/dma-noncoherent.h>
> > >  #include <linux/init.h>
> > > @@ -55,6 +57,31 @@ static void dma_atomic_pool_size_add(gfp_t gfp, size_t
> > > size)
> > >  		pool_size_kernel += size;
> > >  }
> > >  
> > > +static bool cma_in_zone(gfp_t gfp)
> > > +{
> > > +	phys_addr_t end;
> > > +	unsigned long size;
> > > +	struct cma *cma;
> > > +
> > > +	cma = dev_get_cma_area(NULL);
> > > +	if (!cma)
> > > +		return false;
> > > +
> > > +	size = cma_get_size(cma);
> > > +	if (!size)
> > > +		return false;
> > > +	end = cma_get_base(cma) - memblock_start_of_DRAM() + size - 1;
> > > +
> > > +	/* CMA can't cross zone boundaries, see cma_activate_area() */
> > > +	if (IS_ENABLED(CONFIG_ZONE_DMA) && (gfp & GFP_DMA) &&
> > > +	    end <= DMA_BIT_MASK(zone_dma_bits))
> > > +		return true;
> > > +	if (IS_ENABLED(CONFIG_ZONE_DMA32) && (gfp & GFP_DMA32) &&
> > > +	    end <= DMA_BIT_MASK(32))
> > > +		return true;
> > > +	return true;
> > 
> > IIUC this will always return true given a CMA is present. Which reverts to
> > the
> > previous behaviour (previous as in breaking some rpi4 setups), isn't it?
> 
> Was that really what broke the PI?  I'll try to get the split out series
> today, which might have a few more tweaks, and then we'll need to test it
> both on these rpi4 setups and Amits phone.

There was two issues with RPi:
 - Not validating that pool allocated memory was OK for the device
 - Locating all atomic pools in CMA, which doesn't work for all RPi4 devices*,
   and IMO misses the point of having multiple pools.

* With ACPI RPi4 we have CMA located in ZONE_DMA32, yet have an atomic pool
consumer, PCIe, that only wants memory in the [0 3GB] area, effectively needing
ZONE_DMA memory.

Attachment: signature.asc
Description: This is a digitally signed message part