Hi Olivier,
Le ven., mai 21 2021 at 20:15:42 +0200, Olivier Dautricourt
<olivier.dautricourt@xxxxxxxxxx> 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
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
