Jeremy Fitzhardinge <jeremy@goop.org> writes:

quoted text
> Hugh Dickins wrote:
>> As you can see, I'm still groping towards the right answer.
>> The driver probably needs to provide its own backing_dev_info
>> (or point to a suitable default), and its own address_space_ops,
>> and perhaps more (there should be examples elsewhere).  But whether
>> it is actually wrong, or whether I was wrong to mess it up, I've
>> not yet decided.
>>   
>
> My understanding is that the driver is doing something a bit clever:
> it uses the page dirty flags to determine which parts of the
> framebuffer have been written to, and uses that information to
> minimize the amount of stuff that needs to be copied out.  The writes

Yes.

quoted text
> to the pages are not expected to generate actual page faults.
>
> But I haven't really looked at it closely, and I'm not at all familiar
> with the vm at this layer.  I'm not sure how it actually allocates the
> framebuffer memory for example (vmalloc?  incrementally on faults?).

vmalloc()

quoted text
> I'm hoping Markus will leap in, since wrote this stuff.  Or, gasp,
> I'll read the code myself.

The actual cleverness is in fb_defio[*], which was written by Jaya
Kumar (cc'ed).  I merely ripped out the old, somewhat racy cleverness
I inherited from Anthony Liguori (which you can still admire in Xen's
2.6.18 kernel), and switched over to use fb_defio instead.  Because
one instance of clever code is enough.

My understanding of fb_defio's inner workings is rather limited I
fear.  I'm just using it.

Jaya, could you help?

[...]


[*] Documentation/fb/deferred_io.txt drivers/video/fb_defio.c
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