Re: Possible bug when using dmam_alloc_coherent in conjonction with of_reserved_mem_device_release

Olivier Dautricourt
2021-05-22T13:09:57Z

The 05/22/2021 13:31, Paul Cercueil wrote:
> Le sam., mai 22 2021 at 13:42:54 +0200, Olivier Dautricourt
> <olivier.dautricourt@orolia.com> a écrit :
> > Hello Paul,
> >
> > The 05/22/2021 11:28, Paul Cercueil wrote:
> > > Hi Olivier,
> > >
> > >
> > > Le ven., mai 21 2021 at 20:15:42 +0200, Olivier Dautricourt
> > > <olivier.dautricourt@orolia.com> a écrit :
> > > > Hello all,
> > > >
> > > > I am facing a problem when using dmam_alloc_coherent (the managed
> > > > version of dma_alloc_coherent) along with a device-specific
> > > reserved
> > > > memory
> > > > region using the CMA.
> > > >
> > > > My observation is on a kernel 5.10.19 (arm), as i'm unable to test
> > > > the exact
> > > > same configuration on a newer kernel. However it seems that the
> > > > relevent code
> > > > did not change too much since, so i think it's still applicable.
> > > >
> > > >
> > > > ....
> > > > The issue:
> > > >
> > > > I declare a reserved region on my board such as:
> > > >
> > > > mydevice_reserved: linux,cma {
> > > > compatible = "shared-dma-pool";
> > > > reusable;
> > > > size = <0x2400000>;
> > > > };
> > > >
> > > > and start the kernel with cma=0, i want my region to be reserved
> > > to
> > > > my device.
> > > >
> > > > My driver basically does:
> > > >
> > > > probe(dev):
> > > > of_reserved_mem_device_init(dev)
> > > > dmam_alloc_coherent(...)
> > > >
> > > > release(dev):
> > > > of_reserved_mem_device_release(dev)
> > >
> > > You must make sure that whatever is allocated or initialized is
> > > freed
> > > or deinitialized in the reverse order, which is not what will happen
> > > here: release(dev) will be called before the dev-managed cleanups.
> > >
> > > To fix your issue, either use dma_alloc_coherent() and call
> > > dma_free_coherent() in release(), or register
> > > of_reserved_mem_device_release() as a dev-managed cleanup function
> > > (which is what my driver does).
> > >
> > > Cheers,
> > > -Paul
> >
> > as i was saying in my previous mail, i tried to register a devm action
> > to trigger of_reserved_mem_device_release on cleanup but it was still
> > called before dmam_alloc_coherent_memory's cleanup.
> >
> > So my question is: How do you make sure that the managed cleanup
> > routines
> > are executed in the right order ?
>
> And when exactly do you register the devm action?
>
> If you register it right after of_reserved_mem_device_init() and before
> dmam_alloc_coherent(), like in my driver, it should work fine (provided
> your .release doesn't call of_reserved_mem_device_release() itself) and
> you shouldn't have to do anything more than that.

I'm unable to test right now but you must be right, i registered the
devm action after dmam_alloc_coherent, so i suppose that if i change the
order it will work...

However i am still wondering if it is ok for the dmam release
to depend on the reserved mem state to suceed.
I think we should at least issue a warning when __free_from_contiguous
fails.

Olivier

>
> -Paul
>
>
> > Should we have to care about that when using a managed
> > function that belongs to the core ?
> >
> >
> > Olivier
> > >
> > > > On driver detach, of_reserved_mem_device_release will call
> > > > rmem_cma_device_release which sets dev->cma_area = NULL;
> > > > Then the manager will try to free the dma memory allocated in the
> > > > probe:
> > > >
> > > > __free_from_contiguous -> dma_release_from_contiguous ->
> > > > cma_release(dev_get_cma_area(dev), ...);
> > > >
> > > > Except that now dev_get_cma_area will return
> > > > dma_contiguous_default_area
> > > > which is null in my setup:
> > > >
> > > > static inline struct cma *dev_get_cma_area(struct device *dev)
> > > > {
> > > > if (dev && dev->cma_area) // dev->cma_area is null
> > > > return dev->cma_area;
> > > >
> > > > return dma_contiguous_default_area; // null in my setup
> > > > }
> > > >
> > > > and so cma_release will do nothing.
> > > >
> > > > bool cma_release(struct cma *cma, const struct page *pages,
> > > unsigned
> > > > int count)
> > > > {
> > > > unsigned long pfn;
> > > >
> > > > if (!cma || !pages) // cma is NULL
> > > > return false;
> > > >
> > > > __free_from_contiguous will fail silently because it ignores
> > > > dma_release_from_contiguous boolean result.
> > > >
> > > > The driver will be unable to load and allocate memory again
> > > because
> > > > the
> > > > area allocated with dmam_alloc_coherent is not freed.
> > > > ...
> > > >
> > > > So i started to look at drivers using both dmam_alloc_coherent and
> > > > of_reserved_mem_device_release and found this driver:
> > > > (gpu/drm/ingenic/ingenic-drm-drv.c).
> > > > This is why i included the original author, Paul Cercueil, in the
> > > > loop.
> > > >
> > > > Q:
> > > >
> > > > I noticed that Paul used devm_add_action_or_reset to trigger
> > > > of_reserved_mem_device_release on driver detach, is this because
> > > of
> > > > this
> > > > problem that we use a devm trigger here ?
> > > >
> > > > I tried to do the same in my driver, but rmem_cma_device_release
> > > is
> > > > still
> > > > called before dmam_release, is there a way to force the order ?
> > > >
> > > > Is what i described a bug that needs fixing ?
> > > >
> > > >
> > > > Thank you,
> > > >
> > > >
> > > > Olivier
> > > >
> > > >
> > >
> > >
>
>