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

Daniel Vetter daniel at ffwll.ch
Tue Apr 10 08:03:04 PDT 2012

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 at 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 at 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:

Chris, anything you want to add to the wishlist?

Thanks, Daniel
Daniel Vetter
Mail: daniel at ffwll.ch
Mobile: +41 (0)79 365 57 48

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