Re: Possible bug when using dmam_alloc_coherent in conjonction with of_reserved_mem_device_release

[Paul Cercueil <paul@crapouillou.net>]

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.

-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
>>  >
>>  >
>> 
>>