Re: [PATCH] drm: edid: add support for E-DDC

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Aug 23, 2012 at 07:06:53AM -0700, Shirish S wrote:
> Hello Daniel,
> 
> On Thu, Aug 23, 2012 at 1:54 AM, Daniel Vetter <daniel@ffwll.ch> wrote:
> 
> > On Wed, Aug 22, 2012 at 10:52:26AM +0300, Ville Syrjälä wrote:
> > > On Tue, Aug 21, 2012 at 04:28:20PM -0700, Shirish S wrote:
> > > > On Tue, Aug 21, 2012 at 7:56 AM, Ville Syrjälä <
> > > > ville.syrjala@linux.intel.com> wrote:
> > > >
> > > > > On Tue, Aug 21, 2012 at 06:55:53AM -0700, Shirish S wrote:
> > > > Here are my earlier comments on Jean's patch:
> > > > >
> > http://lists.freedesktop.org/archives/dri-devel/2012-February/019069.html
> > > > >
> > > > >
> > > >  If i am not wrong am doing exactly what you have said in you comments.
> > > >
> > > > This seems a bit wrong to me. The spec says that the ack for the
> > > > segment address is "don't care", but for the segment pointer the ack is
> > > > required (when segment != 0).
> > > > The variable segFlags is "dont care for block 0 and 1 wheras".
> > > >
> > > > With I2C_M_IGNORE_NAK we would in fact end up reading segment 0 from a
> > > > non E-DDC display, if we try to read segment != 0 from it. Of course
> > > > we shouldn't do that unless the display lied to us about what extension
> > > > blocks it provides.
> > > >
> > > > So I'm not sure if it would be better to trust that the display never
> > > > lies about the extension blocks, or if we should just assume all E-DDC
> > > > displays ack both segment addr and pointer. The no-ack feature seems
> > > > to there for backwards compatibility, for cases where the host always
> > > > sends the segment addr/pointer even when reading segment 0 (which your
> > > > code doesn't do).
> > > >
> > > > To handle it exactly as the spec says, I2C_M_IGNORE_NAK should be split
> > > > into two flags (one for addr, other for data).
> > > >
> > > > Hence i have split the i2c_msg into 3, segment pointer,offset(addr)
> > > > and data pointer.
> > >
> > > I was referring to the addr and data phases of the segment pointer.
> > > According to the spec the ack for the addr is always optional. But I
> > > suppose no sane device would nak the addr, while acking the data.
> >
> > We've seen those. Really.
> >
> > Which is why the current i915 gmbus driver has a hack to never return a
> > NaK on the first i2c transfer. I guess we should fix this by properly
> > supporting the INGNORE_NAK field in our gmbus driver, and setting that on
> > the addr transfer field, too.
> >
> > Thanks for the comment, so are you ok with the current logic?
> 
> 
> > I concure with Ville that sending the segment i2c message only when we
> > actually need it, and not unconditionally. DDC is way to broken and
> > claiming that the spec says otherwise doesn't fix all the existing bad hw.
> >
> 
> Agreed, so do you want me to post another patch, in which i add a function
> only
> if the number of blocks is more than 2?
> Also i had some mistake in the patch set 1, hence i updated it.

I think adding the IGNORE_NAK on the addr i2c transaction would help
unconditionally - like I've said we've seen monitors that suggest that
this is required. And yeah, I think we should send the E-DDC segment
number only if the basic edid block indicates that more than 2 blocks are
availble (and again with IGNORE_NAK, just for paranoia's sake).


> Kindly have a look!

Will do.

Yours, Daniel
-- 
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48