[Q] comedi: vmap of the async->prealloc_buf

[H Hartley Sweeten]

On Friday, December 21, 2012 12:01 AM, Dan Carpenter wrote:
> On Thu, Dec 20, 2012 at 10:36:39PM +0000, Ian Abbott wrote:
>> On 20/12/12 17:09, H Hartley Sweeten wrote:
>>>Hello all,
>>>
>>>I have a question...
>>>
>>>In the comedi core, the asynchronous 'prealloc_buf' is vmap'ed after
>>>the page list has been allocated. The code in question looks like this:
>>>
>>>			async->prealloc_buf =
>>>#ifdef PAGE_KERNEL_NOCACHE
>>>				vmap(pages, n_pages, VM_MAP, PAGE_KERNEL_NOCACHE);
>>>#else
>>>				vmap(pages, n_pages, VM_MAP, PAGE_KERNEL);
>>>#endif
>>>
>>>Is the #ifdef something left over from an older kernel?
>>>
>>>Isn't PAGE_KERNEL_NOCACHE always defined?
>> 
>> Unfortunately not; it's arch dependent.  Neither ARM nor PowerPC
>> define it, for example.
>
> Power PC has _PAGE_NO_CACHE and the DRM subsystem uses that like
> PAGE_KERNEL_NOCACHE.  For example PAGE_AGP is define in one of these
> ways:
>
> #define PAGE_AGP PAGE_KERNEL_NOCACHE
> #define PAGE_AGP __pgprot(_PAGE_KERNEL | _PAGE_NO_CACHE)  <- only on powerpc
> #define PAGE_AGP PAGE_KERNEL
>
> I don't really know what's going on here though...
>
> comedia and AGP are the only drivers which use PAGE_KERNEL_NOCACHE.

Yeah, I noticed it's not used very much.

I was hoping to either remove its use or just use by default. Either way it would get
rid of the #ifdef.

But, I'm not sure what's going on either so for not I guess it needs to stay.

Regards,
Hartley