Re: [PATCH v2 27/27] drm/tegra: Add Tegra114 gr2d support

[Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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On Mon, Oct 14, 2013 at 12:14:47PM -0600, Stephen Warren wrote:
> On 10/14/2013 08:00 AM, Thierry Reding wrote:
> > On Mon, Oct 14, 2013 at 08:58:34AM +0300, Terje Bergström wrote:
> >> On 12.10.2013 01:43, Stephen Warren wrote:
> >>> On 10/07/2013 02:34 AM, Thierry Reding wrote:
> >>>> The gr2d hardware in Tegra114 is compatible with that of
> >>>> Tegra20 and Tegra30. No functionaly changes are required.
> >>> Similarly here, if the HW is 100% backwards-compatible, there's
> >>> no need to add compatible values to the driver.
> >> 
> >> We've used this mechanism for attaching a per-hw-version data
> >> structure in match table to accomodate differences in how the
> >> hardware is power gated, reset, booted, some per-soc performance
> >> related changes etc. It's also used in staging features for new
> >> chips, such as disabling power features when they're not
> >> working/verified yet.
> >> 
> >> Upstream driver is not yet in a state where that is relevant.
> >> 
> >> With this, would we still be able to do that with match table? It
> >> sounds like we could, because we can still (even with multiple
> >> compatible properties) add separate entries in match table and I
> >> guess the compatible properties matched in order.
> > 
> > Yes, as long as the device tree files includes the most specific
> > value in the compatible this should still be possible. So we'd have
> > this:
> > 
> > gr2d@54140000 { compatible = "nvida,tegra114-gr2d",
> > "nvidia,tegra20-gr2d"; ... };
> > 
> > and the driver will match on "nvidia,tegra20-gr2d" if the more
> > specific "nvidia,tegra114-gr2d" is not there. When the driver is
> > updated to support Tegra114 specific functionality, then a more
> > specific entry can be added to the compatible table to handle it.
> 
> True, but the DT fragment above is also only accurate /if/ a driver
> that only knows about "nvidia,tegra20-gr2d" can operate 100% of the
> features in Tegra20 HW on Tegra114 HW forever.

Yes, but given that we also list "nvidia,tegra114-gr2d" in the property
it will be possible to add that to the driver when new functionality is
implemented and immediately take advantage of it on Tegra114 hardware
with an old device tree file which has both compatible values.

Thierry

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