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

[Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>]

On 10/14/2013 11:51 PM, Terje Bergström wrote:
> On 14.10.2013 21:14, Stephen Warren wrote:
>> On 10/14/2013 08:00 AM, Thierry Reding wrote:
>>> 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.
> 
> I don't know of any hardware incompatibility. The only difference is
> related to something not directly to 2D. We moved host1x away from the
> power domain, so in Tegra114 we're able to power gate 2D and EPP. The
> DVFS tables are also different.
> 
> I'd say say adding the compatible property "nvidia,tegra20-gr2d" for
> Tegra114's 2D is accurate and we're able to use the match table to drive
> any SW policy differences.

The compatible value shouldn't be used for SW policy differences. DT is
about HW, not SW policy. You have to decide: either the HW is 100%
backwards-compatible, or it's not. You can't decide that the HW is
compatible, but then require different compatible values.