Re: ALSA: pcm: use dma_can_mmap() to check if a device supports dma_mmap_*

On Mon, 04 Nov 2019 09:49:43 +0100,
Greg Kroah-Hartman wrote:

On 11/4/19 9:32 AM, Greg Kroah-Hartman wrote:

quoted

On 11/4/19 9:27 AM, youling 257 wrote:
 > This driver

https://android.googlesource.com/kernel/common/+/refs/heads/android-mainline/drivers/usb/gadget/function/f_audio_source.c 

quoted

 >

The driver is broken and needs to be fixed.  Please feel free to submit
patches to AOSP to do so as you can trigger this easily.

Hm, maybe the driver isn't broken...

It is :)

snd_pcm_lib_preallocate_pages_for_all() is called with
SNDRV_DMA_TYPE_DEV set, so that should be fine,

That's the cause.  It passes NULL to the device object, which is
incorrect in anyway.  I guess this used to work casually just because
x86 accepts the NULL device object as if an ISA device.

and the only other
buffer I can see allocate here is with a call to
snd_pcm_lib_alloc_vmalloc_buffer() which _should_ be ok, right?

If the driver allocates the buffer via vmalloc (and the snd_* helper),
it shouldn't do preallocate in that way.  That is, the correct fix
would  be simply to drop snd_pcm_lib_preallocate_pages_for_all() call
from the driver code.

Also, a quick skimming showed that the driver needs to set
snd_pcm_lib_get_vmalloc_page to the snd_pcm_ops.page ops.

I don't see any buffers coming off the stack here, unless the gadget
controller is the one creating them?

That's the code before actually allocating the buffer itself.  It
checks the availability of mmap support on the architecture, and the
helper code assumed a proper device object passed there via the
preallocation helper.


thanks,

Takashi