Re: [PATCH v2] mm: dmapool: use provided gfp flags for all dma_alloc_coherent() calls

Soeren Moch
2013-01-23T14:39:21Z

On 22.01.2013 19:13, Arnd Bergmann wrote:
> On Monday 21 January 2013, Greg KH wrote:
>>>
>>> I don't know a lot about USB, but I always assumed that this was not
>>> a normal condition and that there are only a couple of URBs per endpoint
>>> used at a time. Maybe Greg or someone else with a USB background can
>>> shed some light on this.
>>
>> There's no restriction on how many URBs a driver can have outstanding at
>> once, and if you have a system with a lot of USB devices running at the
>> same time, there could be lots of URBs in flight depending on the number
>> of host controllers and devices and drivers being used.

I only use one host controller and (in this test) two usb devices with
the same driver.

> Ok, thanks for clarifying that. I read some more of the em28xx driver,
> and while it does have a bunch of URBs in flight, there are only five
> audio and five video URBs that I see simultaneously being submitted,
> and then resubmitted from their completion handlers. I think this
> means that there should be 10 URBs active at any given time in this
> driver, which does not explain why we get 256 allocations.

I think the audio part of the em28xx bridge is not used in my DVB tests.

Are there other allocations from orion-ehci directly? Maybe something
special for isochronous transfers (since there is no problem with my
other dvb sticks using bulk transfers)?

> I also noticed that the initial submissions are all atomic but don't
> need to, so it may be worth trying the patch below, which should also
> help in low-memory situations. We could also try moving the resubmission
> into a workqueue in order to let those be GFP_KERNEL, but I don't think
> that will help.

I built a linux-3.7.4 with the em28xx patch and both of your dma-mapping.c
patches. I still see the
ERROR: 1024 KiB atomic DMA coherent pool is too small!

Soeren