Re: [RFC v1] drm/i915: Add Exec param to control data port coherency.

["Lis, Tomasz" <tomasz.lis@intel.com>]

On 2018-03-19 13:43, Chris Wilson wrote:
> Quoting Tomasz Lis (2018-03-19 12:37:35)
>> The patch adds a parameter to control the data port coherency functionality
>> on a per-exec call basis. When data port coherency flag value is different
>> than what it was in previous call for the context, a command to switch data
>> port coherency state is added before the buffer to be executed.
> So this is part of the context? Why do it at exec level?

It is part of the context, stored within HDC chicken bit register.
The exec level was requested by the OCL team, due to concerns about 
performance cost of context setparam calls.

>   If exec level
> is desired, why not whitelist it?
> -Chris

If we have no issue in whitelisting the register, I'm sure OCL will 
agree to that.
I assumed the whitelisting will be unacceptable because of security 
concerns with some options.
The register also changes its position and content between gens, which 
makes whitelisting hard to manage.

Main purpose of chicken bit registers, in general, is to allow work 
around for hardware features which could  be buggy or could have 
unintended influence on the platform.
The data port coherency functionality landed there for the same reasons; 
then it twisted itself in a way that we now need user space to switch it.
Is it really ok to whitelist chicken bit registers?
-Tomasz

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