[Intel-gfx] Design review request: DRM color manager

[Sharma, Shashank]

Thanks again David, 
Comments inline. 

Regards
Shashank
-----Original Message-----
From: David Herrmann [mailto:dh.herrmann at gmail.com] 
Sent: Tuesday, April 22, 2014 5:10 PM
To: Sharma, Shashank
Cc: intel-gfx at lists.freedesktop.org; dri-devel at lists.freedesktop.org; Ville Syrjälä; Thierry Reding; Alex Deucher; Sean Paul; robdclark at gmail.com; Mukherjee, Indranil; Jindal, Sonika; Korjani, Vikas; Shankar, Uma; Cn, Ramakrishnan
Subject: Re: Design review request: DRM color manager

Hi

On Tue, Apr 22, 2014 at 12:21 PM, Sharma, Shashank <shashank.sharma at intel.com> wrote:
> 1) Why do you register only a single property? Why not register a separate property for each color-correction that is available? This way you can drop the property-id and use the high-level DRM-prop IDs/names.
>>> That’s the whole idea of color manager. If we keep on creating properties for each color correction, there would be a big list and a lot of properties will be exposed. Instead one common blob which can represent all the properties, correction values and identifiers. It would be easy to club with atomic modeset kind-of designs also I believe.

Where is the difference? With one _well-defined_ property for each type we simply move the identification one level up. With your approach you just move the type-id one level down into the blob.

Or in other words: Where is the difference between calling
SetProperty() n-times, or calling it once but with a parameter describing n-properties? With atomic-modesetting we can set as many properties as we want and make the kernel apply them atomically.

>>> Actually we also do not want to populate the property space also, as if there are 10 color correction methods possible for a hardware, we might end up listing 10 properties.  And there won't be common properties across all the hardwares also. For example, Hardware A can have properties X Y Z but Hardware B can have W X and Z. This will make the property space inconsistent. But if we provide one common interface which will cover for all the properties, for all the hardwares in a single blob. The driver will dynamically register its property, in its own preferred name. A get_prop() will always list down all the supported color property by this hardware and driver.   

> 2) What is the CRTC-ID for? DRM properties can be set on a specific CRTC and/or plane. Isn't that enough information for the driver?
>>> This is to make it HW agonist. Actually that's CRTC ID / Plane ID / PIPE ID / all together an identifier. For example if I want to set gamma correction for sprite planes only, not on primary plane or pipe level, on VLV, its possible. This gives me flexibility to mention fine-tuned correction even in a CRTC.  The driver's .enable method can take decision on this identifier based on the hardware capabilities.

Yeah, but I meant the drmModeObjectSetProperty() ioctl already tales a CRTC/Plane/Connector ID. So why duplicate that information in the blob? And more importantly, what happens if you call
drmModeObjectSetProperty() on a plane but specify a CRTC ID in the blob? Seems weird to me to support such setups.

>>> The design is to register color-manager as a CRTC property, to make it consistent, and then give the fine tuning via this identifier byte. 
Else we have to keep track of this in userspace, that which property is valid for which extent. For example, Hue and saturation correction, on VLV, can be applied on Sprite planes only(not on primary plane). So we have to send a plane as an object here. 
Rather in color manager case, we will always send the CRTC as an object to IOCTL, but will specify SPRITE_PLANE as identifier. Does this sound less weird now  :) ? 

> 3) Please document the payload for each of the properties you define.
> If the property is a blob, there is no reason to make the properties generic. User-space requires a common syntax across all drivers, otherwise, it cannot make use of generic properties and you should use driver-dependent properties instead.
>>> Can you please elaborate a bit more ? I believe that a blob is a superset of single and multi-valued properties. So we can use the byte defined for <no of correction bytes> and specify both single value and multi value correction using the same interface, >> method and protocol.  So any userspace can just follow this, any can give commands to any driver.

Well, your document doesn't describe the payload at all. I just wanted a description of what kind of information is expected. Number of arguments, argument size, argument types, argument description.. and so on.
>>>> Sure, I will further document it very clearly about arguments and descriptions. Actually we have discussed the protocol in the color EDID section, which tells us about the 4 byte protocol and expectation, but that’s elementary. 

> 4) We have a tuple-type for properties. So in case you only need 32bit payloads for a given property, you can combine enable/disable and value in a single 64bit property.
>>> But properties like CSC and Gamma correction need multiple correction values, up to 256 32-bit values. For this we need more no of values. AM I getting it right ?
Sure.

Thanks
David