CMA question

[Johannes Thoma]

Dear Greg,

Thank you for your quick response.

Am 26.02.17 um 17:31 schrieb Greg KH:
> On Sun, Feb 26, 2017 at 05:29:05PM +0100, Johannes Thoma wrote:
>> Dear Kernel hackers,
>>
>> As far as I understood CMA (contiguous memory allocation) memory is used
>> for other purposes as long it isn't requested via cma_alloc() by a
>> driver. cma_alloc then tries to free the memory by relocating it and
>> returns the contiguous area.
>>
>> I have a case where cma_alloc() sometimes fails to relocate the memory
>> which causes my driver (a GPU driver) to fail starting up (the driver is
>> started when the system is running for a while). Is there a way to
>> prevent the CM allocator to use the memory for any other purpose? Or
>> should I better use another mechanism to allocate  contiguous memory at
>> boot time (is there a framework for doing so)?
>
> If at boot time you don't have enough memory, something is really wrong
> and your driver shouldn't work, that's to be expected.  Try starting it
> earlier in the boot process.
>
Maybe that is the problem. The call to cma_alloc() happens in a
driver-specific API function which is called when my application starts
(at uptime 30 seconds it always succeeded (at least I didn't observe any
failures), however when I restart the application later and call
cma_alloc() again (the memory was freed meanwhile) it sometimes fails).

> Do you have a pointer to your driver anywhere so we could see if you are
> doing something odd with the cma interface?
>

The driver is part of the imx kernel from boundary devices and can be found here:

https://github.com/boundarydevices/linux-imx6/blob/boundary-imx_4.1.15_2.0.0_ga/drivers/mxc/gpu-viv

The call to dma_alloc_writecombine (which eventually calls cma_alloc() if I am not
completely wrong) can be found in this file:

https://github.com/boundarydevices/linux-imx6/blob/boundary-imx_4.1.15_2.0.0_ga/drivers/mxc/gpu-viv/hal/os/linux/kernel/allocator/freescale/gc_hal_kernel_allocator_cma.c

Note that I didn't write the driver, I am just using it (it seems to be a port from
another OS to Linux to me).

As you pointed out the solution would be to allocate the memory earlier in the
boot process, by modifying the driver. I will try that in the next few days and
let you know the result.

Best,

- Johannes