Re: [PATCH/RFC 2.6.20-rc4 1/1] fbdev,mm: hecuba/E-Ink fbdev driver

["Jaya Kumar" <jayakumar.lkml@gmail.com>]

On 1/11/07, Andrew Morton <akpm@osdl.org> wrote:
> That's all very interesting.
>
> Please don't dump a bunch of new implementation concepts like this on us
> with no description of what it does, why it does it and why it does it in
> this particular manner.

Hi Andrew,

Actually, I didn't dump without description. :-) I had posted an RFC
and an explanation of the design to the lists. Here's an archive link
to that post. http://marc.theaimsgroup.com/?l=linux-kernel&m=116583546411423&w=2
I wasn't sure whether to include that description with the patch email
because it was long.

> From that email:
---
This is there in order to hide the latency
associated with updating the display (500ms to 800ms). The method used
is to fake a framebuffer in memory. Then use pagefaults followed by delayed
unmaping and only then do the actual framebuffer update. To explain this
better, the usage scenario is like this:

- userspace app like Xfbdev mmaps framebuffer
- driver handles and sets up nopage and page_mkwrite handlers
- app tries to write to mmaped vaddress
- get pagefault and reaches driver's nopage handler
- driver's nopage handler finds and returns physical page ( no
 actual framebuffer )
- write so get page_mkwrite where we add this page to a list
- also schedules a workqueue task to be run after a delay
- app continues writing to that page with no additional cost
- the workqueue task comes in and unmaps the pages on the list, then
 completes the work associated with updating the framebuffer
- app tries to write to the address (that has now been unmapped)
- get pagefault and the above sequence occurs again

The desire is roughly to allow bursty framebuffer writes to occur.
Then after some time when hopefully things have gone quiet, we go and
really update the framebuffer. For this type of nonvolatile high latency
display, the desired image is the final image rather than intermediate
stages which is why it's okay to not update for each write that is
occuring.
---

> What is the "theory of operation" here?
>
> Presumably this is a performance optimisation to permit batching of the
> copying from user memory into the frambuffer card?  If so, how much
> performance does it gain?

Yes, you are right. Updating the E-Ink display currently requires
about 500ms - 800ms. It is a non-volatile display and as such it is
typically used in a manner where only the final image is important. As
a result, being able to avoid the bursts of IO associated with screen
activity and only write the final result is attractive.

I have not done any performance benchmarks. I'm not sure exactly what
to compare. I imagine in one case would be using write() to deliver
the image updates and the other case would be mmap(), memcpy(). The
latter would win because it's hiding all the intermediate "writes".

> I expect the benefit will be large, and could be increased if you were to
> add a small delay between first-touch and writeback to the display.  Let's
> talk about that a bit.

Agreed. Though I may be misunderstanding what you mean by first-touch.
Currently, I do a schedule_delayed_work and leave 1s between when the
page_mkwrite callback indicating the first touch is received and when
the deferred IO is processed to actually deliver the data to the
display. I picked 1s because it rounds up the display latency. I
imagine increasing the delay further may make it miss some desirable
display activity. For example, a slider indicating progress of music
may be slower than optimal. Perhaps I should make the delay a module
parameter and leave the choice to the user?

> Is the optimisation applicable to other drivers?  If so, should it be
> generalised into library code somewhere?

I think the deferred IO code would be useful to devices that have slow
updates and where only the final result is important. So far, this is
the only device I've encountered that has this characteristic.

> I guess the export of page_mkclean() makes sense for this application.
>
> The use of lock_page_nosync() is wrong.  It can still sleep, and here it's
> inside spinlock.  And we don't want to export __lock_page_nosync() to
> modules.  I suggest you convert the list locking here to a mutex and use
> lock_page().

Oops, sorry about that. I will correct it.

Thanks,
jayakumar
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