Question: how to copy to user space from a tasklet

Question: how to copy to user space from a tasklet

[Tom Brown]

[Sorry to have to ask this here - I can't find any driver-specific 
newsgroup or mailing lists - are there any??]

I have a driver which needs to copy device data direct to user space 
from a tasklet. I need to do this because the device is continuously 
DMA'ing into user space, with no user intervention, so the user isn't 
waiting for data. In other words, I don't have any user-associated 
process to wake up.

This means that I have to somehow copy the data directly to the user 
from the tasklet, without sleeping, and without direct access to user 
context. Can someone tell me how to do this?

Thanks

Tom

Re: Question: how to copy to user space from a tasklet

[Yong Zhang]

On Thu, Oct 14, 2010 at 11:43:16AM +0100, Tom Brown wrote:
> [Sorry to have to ask this here - I can't find any driver-specific
> newsgroup or mailing lists - are there any??]
> 
> I have a driver which needs to copy device data direct to user space
> from a tasklet. I need to do this because the device is continuously
> DMA'ing into user space, with no user intervention, so the user
> isn't waiting for data. In other words, I don't have any
> user-associated process to wake up.
> 
> This means that I have to somehow copy the data directly to the user
> from the tasklet, without sleeping, and without direct access to
> user context. Can someone tell me how to do this?

Are you seeking for probe_kernel_read/write(include/linux/uaccess.h)?

Thanks,
Yong

> 
> Thanks
> 
> Tom
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

Re: Question: how to copy to user space from a tasklet

[Stefan Richter]

Tom Brown wrote:
> [Sorry to have to ask this here - I can't find any driver-specific
> newsgroup or mailing lists - are there any??]

linux-newbie perhaps.

> I have a driver which needs to copy device data direct to user space
> from a tasklet.

One way would be to use mmap() to allocate the DMA buffer and insert it into
the user address space before DMA commences.  An additional poll/read/write
based protocol or so can be used for buffer management during DMA.
-- 
Stefan Richter
-=====-==-=- =-=- -====
http://arcgraph.de/sr/

Re: Question: how to copy to user space from a tasklet

[Arnd Bergmann]

On Friday 15 October 2010 07:57:59 Stefan Richter wrote:
> Tom Brown wrote:
> > [Sorry to have to ask this here - I can't find any driver-specific
> > newsgroup or mailing lists - are there any??]
> 
> linux-newbie perhaps.
> 
> > I have a driver which needs to copy device data direct to user space
> > from a tasklet.
> 
> One way would be to use mmap() to allocate the DMA buffer and insert it into
> the user address space before DMA commences.  An additional poll/read/write
> based protocol or so can be used for buffer management during DMA.

Agreed, that would be best, it's a lot easier than the way Tom describes
the driver to work today.

Just for completeness: doing DMA to random user address would require the
process to register the area using some home grown interface, so that the
driver can do get_user_pages/kmap on all of them and subsequently pass
around an array of pages, while keeping track of ulimits and coherency
with the existing kernel mappings. Don't do that.

	Arnd

Re: Question: how to copy to user space from a tasklet

[Tom Brown]

On 15/10/2010 06:57, Stefan Richter wrote:

> One way would be to use mmap() to allocate the DMA buffer and insert it into
> the user address space before DMA commences.  An additional poll/read/write
> based protocol or so can be used for buffer management during DMA.

Thanks, think I'm there:

1 - The user calls mmap() with a specific offset to flag that he wants a 
DMA buffer

2 - I then 'vmalloc' the buffer, and call 'setPageReserved' for each page

I then step over each page, and:

3 - call 'vmalloc_to_pfn' to get a PFN for each page, and then

4 - call 'remap_page_range' for that page (I instead call 
'remap_pfn_range' for kernels >= 2.6.10, but I'm on 2.6.9)

This gives me the buffer, and the interrupt handler then just uses 
'memcpy' to copy data to the user.

I've got to say, this was incredibly hard work. I can't find any useful 
documentation and it can take a couple of hours on Google to write a few 
lines of code. LDD is next to useless. There's no 'vmalloc_to_pfn' on 
2.6.9, so I copied an old deprecated one from 2.6.10 (which was also in 
a kernel prior to 2.6.9, I think). I'm not yet convinced that all this 
is working; I think I've seen one instance when the 'memcpy' to the 
buffer didn't work, so I think there might potentially be a problem with 
setting the reserved bit (and isn't the reserved bit deprecated in 
2.6.15??)

On the other stuff, thanks Yong for the 'probe_kernel_' suggestion, but 
I can't find any documentation on it.

linux-newbie only had one thread on it for the entire month when I 
checked a couple of days ago, apart from a couple of 419 scams.

Thanks -

Tom

Re: Question: how to copy to user space from a tasklet

[Stefan Richter]

Tom Brown wrote:
> Thanks, think I'm there:
> 
> 1 - The user calls mmap() with a specific offset to flag that he wants a
> DMA buffer
> 
> 2 - I then 'vmalloc' the buffer, and call 'setPageReserved' for each page
> 
> I then step over each page, and:
> 
> 3 - call 'vmalloc_to_pfn' to get a PFN for each page, and then
> 
> 4 - call 'remap_page_range' for that page (I instead call
> 'remap_pfn_range' for kernels >= 2.6.10, but I'm on 2.6.9)
> 
> This gives me the buffer, and the interrupt handler then just uses
> 'memcpy' to copy data to the user.

I would have thought of a bunch of alloc_page() and vm_insert_page() but I
don't know if vm_insert_page exists in 2.6.9 and if it honors user limits.

Plus dma_map_page() to get zero-copy operation with a DMA capable device.
-- 
Stefan Richter
-=====-==-=- =-=- =--=-
http://arcgraph.de/sr/

Re: Question: how to copy to user space from a tasklet

[Tom Brown]

On 18/10/2010 18:01, Stefan Richter wrote:
> Tom Brown wrote:
>> Thanks, think I'm there:
>>
>> 1 - The user calls mmap() with a specific offset to flag that he wants a
>> DMA buffer
>>
>> 2 - I then 'vmalloc' the buffer, and call 'setPageReserved' for each page
>>
>> I then step over each page, and:
>>
>> 3 - call 'vmalloc_to_pfn' to get a PFN for each page, and then
>>
>> 4 - call 'remap_page_range' for that page (I instead call
>> 'remap_pfn_range' for kernels>= 2.6.10, but I'm on 2.6.9)
>>
>> This gives me the buffer, and the interrupt handler then just uses
>> 'memcpy' to copy data to the user.
>
> I would have thought of a bunch of alloc_page() and vm_insert_page() but I
> don't know if vm_insert_page exists in 2.6.9 and if it honors user limits.

It turns out that I'm getting occasional

"BUG: unable to handle kernel paging request at virtual address 
...[buffer address]"

oops when the driver attempts to 'memcpy' to the buffer, so I think 
'setPageReserved' is probably not the right thing to do.  This is 
strange - it's almost as if 'setPageReserved' stops the page being 
swapped out, but the kernel doesn't know this.

I'll look into alloc_page/vm_insert_page instead.

Thanks -

Tom