input thread [was Re: [PATCH v2] input: constify valuators passed in by input drivers.]

[Adam Jackson]

On Fri, 2010-08-13 at 08:53 -0700, Aaron Plattner wrote:
> On Fri, Aug 13, 2010 at 08:09:24AM -0700, Adam Jackson wrote:
> > ShmPutImage is a bit better in that it elides the socket copies, but
> > that last memcpy or DMA still has to fire, and it still completes
> > synchronously; the server won't advance to the next request until it's
> > done.
> 
> I know it's nitpicking, but this is false at least for our driver.
> {Shm,}PutImage is pipelined and you can have a potentially large number of
> them in flight at a time.

I assume you do this by hooking out the dispatch for ShmPutImage, since
otherwise you're racing with sending the completion event.  (Or by
memcpying the shm segment aside I guess, but that seems like losing.)

> > And of course {Shm,}GetImage have all the same problems.
> 
> GetImage obviously does have to wait until the DMA is complete, so it's
> worse than PutImage.

Enh.  We have a protocol buffer allocated for the DMA to get into, we
could start the DMA, sleep the client, and then finish protocol when it
completes.  Which isn't really different from the PutImage case.

GetImage performance is a lost cause though, the protocol buffer is like
64k, anyone seriously doing screen scrapes for performance should be
doing ShmGetImage.

- ajax
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