Re: [PATCH 4/8 v7] drm/i915/intel_i2c: use WAIT cycle, not STOP

[Chris Wilson <chris@chris-wilson.co.uk>]

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On Tue, 10 Apr 2012 17:03:04 +0200, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Tue, Apr 10, 2012 at 06:56:15PM +0800, Daniel Kurtz wrote:
> > On Tue, Apr 10, 2012 at 6:41 PM, Daniel Vetter <daniel@ffwll.ch> wrote:
> > > On Tue, Apr 10, 2012 at 12:37:46PM +0200, Daniel Vetter wrote:
> > >> On Fri, Mar 30, 2012 at 07:46:39PM +0800, Daniel Kurtz wrote:
> > >> > The i915 is only able to generate a STOP cycle (i.e. finalize an i2c
> > >> > transaction) during a DATA or WAIT phase.  In other words, the
> > >> > controller rejects a STOP requested as part of the first transaction in a
> > >> > sequence.
> > >> >
> > >> > Thus, for the first transaction we must always use a WAIT cycle, detect
> > >> > when the device has finished (and is in a WAIT phase), and then either
> > >> > start the next transaction, or, if there are no more transactions,
> > >> > generate a STOP cycle.
> > >> >
> > >> > Note: Theoretically, the last transaction of a multi-transaction sequence
> > >> > could initiate a STOP cycle.  However, this slight optimization is left
> > >> > for another patch.  We return -ETIMEDOUT if the hardware doesn't
> > >> > deactivate after the STOP cycle.
> > >> >
> > >> > Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
> > >>
> > >> I've re-read gmbus register spec and STOP seems to be allowed even in the
> > >> first cycle. Does this patch solve an issue for you? If not, I prefer we
> > >> just drop it.
> > 
> > STOP does not work in the first cycle, hence the patch.
> 
> Ok, I've picked this patch up and extended the comment a bit to that
> effect. Just to avoid anyone else trying to 'fix' things because bspec
> sounds like it should work.
> 
> I've also picked up the other patches safe for the last one, thanks a lot
> for digging through the gmbus code and fixing it all up.
> 
> Now can I volunteer you for a (hopefully) last set of gmbus patches?
> Afaics there are a few small things left to fix:
> - zero-length reads can blow up the kernel, like zero-length writes could.
>   See: https://bugs.freedesktop.org/show_bug.cgi?id=48269
> - Chris Wilson suggested on irc that we should wait for HW_READY even for
>   zero-length writes (and also reads), currently we don't.
> - atm the debug output is too noisy. I think we can leave the fallback to
>   gpio bitbanging at info (or maybe error) level, but all the other
>   messages should be tuned down to DRM_DEBUG_KMS - these can easily be hit
>   when userspace tries to probe the i2c with nothing connected or if the
>   driver code tries to do the same. See:
>   https://bugs.freedesktop.org/show_bug.cgi?id=48248
> 
> Chris, anything you want to add to the wishlist?

The last major item on the wishlist is solving how to drive the SDVO i2c
over gmbus. I think it is just a matter of massaging in the channel
switch as msg[0].
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre