[Intel-gfx] [PATCH] drm/i915: RMW register cycles considered evil

[Dave Gordon]

On 06/07/15 13:50, Ville Syrjälä wrote:
> On Mon, Jul 06, 2015 at 02:42:02PM +0200, Daniel Vetter wrote:
>> Especially for workarounds which is stuff that's almost impossible to
>> verify: The initial state from the firmware on boot-up and after
>> resume could be different, which will hide bugs when we do an RMW
>> cycle.
>
> If you're really worried about that then we should then explicitly
> initialize all the registers that might affect stuff.
>
> For a bunch of GT registers we could just do a GPU reset at driver
> init. That that won't help with UCGCTL and such.
>
> I'm also worried that if we don't use RMWs for early parts, the hardware
> folks may still change the default for some ofhte other bits, and then
> we end up clobbering those.

In other drivers, I've found a good pattern to be:

1.	during driver load, snapshot (just once) anything that the BIOS
	may have programmed that we may need later
2.	then reset the h/w and reprogram it totally to our preferred
	values, which may to a greater or lesser degree be derived from
	the saved BIOS settings
3.	during unload, reset the h/w again and reprogram it with the
	BIOS settings

2a.	resume is just like load, except we don't need or want to
	capture the BIOS settings first
3a.	suspend is like unload, except in some cases the BIOS values
	might need to be tweaked when writing them back in order to
	ensure the device doesn't generate further activity.

Step 1 ensures that we don't lose useful settings where the BIOS knows 
better than the driver what the good values are.
Step 2/2a. ensures that the device starts out in a well-defined state, 
regardless of how comprehensively (or badly) the BIOS has set it up.
It seems a reasonable compromise between taking advantage of good BIOSes 
while not getting too much hassle from broken ones.

.Dave.