On Fri, Dec 05, 2014 at 02:28:18PM -0800, Sean Paul wrote:
> On Fri, Dec 5, 2014 at 2:17 PM, Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> wrote:
> 
> > On Fri, Dec 05, 2014 at 01:48:42PM -0800, Sean Paul wrote:
> > > On Tue, Dec 2, 2014 at 5:40 PM, David Ung <davidu-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org> wrote:
> > >
> > > > Fix DSI PHY HS Trail setting on Tegra.
> > > > It is found that HS Trail is off -12% during compliance testing
> > > >
> > > >
> > > [this time from the correct email address]
> > >
> > > Our timing tests pass now.
> > >
> > > Tested-by: Sean Paul <seanpaul-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
> >
> > Can describe what kinds of timing tests these are? I obviously lack the
> > kind of equipment for this, but I'd still like to know how you get these
> > results.
> >
> >
> Hi Thierry,
> The test that was failing was the THS-TRAIL test. You can find the details
> at [1], test is 8.1.13. The time was 7.7ns too short (or 12%, as David
> stated).
> 
> I'm not sure how David arrived at the value he did to extend HS-TRAIL, but
> that particular test is now passing.

From reading the document that you linked to and comparing to the D-PHY
specification (version 1.2, Table 14, Page 40), the proper formula for
the HSTrail parameter would be:

	max(n * 8 * period, 60 + n * 4 * period)

I suspect that the additional 3 * period * 8 in this patch is derived
from the note about "n", which is 1 for forward-direction high-speed
mode and 4 for reverse-direction high-speed mode. Translating this to
the code, the correct line would really be:

	timing->hstrail = max(4 * 8 * period, 60 + 4 * 4 * period);

Since the "n" parameter appears in the second term, too. Also this is
not Tegra-specific after all, so mipi_dphy_timing_get_default() indeed
is the right place for it.

How about the attached patch?

Thierry