Re: [PATCH 1/1] drm/i915: allow to select rc6 modes via kernel parameter

[Ben Widawsky <ben@bwidawsk.net>]

On 02/11/2012 08:23 PM, Eugeni Dodonov wrote:
> This allows to select which rc6 modes are to be used via kernel parameter,
> via a bitmask parameter. E.g.:
> 
> - to enable rc6, i915_enable_rc6=1
> - to enable rc6 and deep rc6, i915_enable_rc6=3
> - to enable rc6 and deepest rc6, use i915_enable_rc6=5
> - to enable rc6, deep and deepest rc6, use i915_enable_rc6=7
> 
> Please keep in mind that the deepest RC6 state really should NOT be used
> by default, as it could potentially worsen the issues with deep RC6. So do
> enable it only when you know what you are doing. However, having it around
> could help solving possible future rc6-related issues and their debugging
> on user machines.
> 
> Note that this changes behavior - previously, value of 1 would enable both
> RC6 and deep RC6. Now it should only enable RC6 and deep/deepest RC6
> stages must be enabled manually.
> 
> v2: address Chris Wilson comments and clean up the code.
> 
> Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_drv.c      |    6 +++++-
>  drivers/gpu/drm/i915/i915_drv.h      |   21 +++++++++++++++++++++
>  drivers/gpu/drm/i915/intel_display.c |   20 ++++++++++++++++----
>  3 files changed, 42 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
> index a1103fc..b7a91db 100644
> --- a/drivers/gpu/drm/i915/i915_drv.c
> +++ b/drivers/gpu/drm/i915/i915_drv.c
> @@ -66,7 +66,11 @@ MODULE_PARM_DESC(semaphores,
>  int i915_enable_rc6 __read_mostly = -1;
>  module_param_named(i915_enable_rc6, i915_enable_rc6, int, 0600);
>  MODULE_PARM_DESC(i915_enable_rc6,
> -		"Enable power-saving render C-state 6 (default: -1 (use per-chip default)");
> +		"Enable power-saving render C-state 6. "
> +		"Different stages can be selected via bitmask values "
> +		"(0 = disable; 1 = enable rc6; 2 = enable deep rc6; 4 = enable deepest rc6). "
> +		"For example, 3 would enable rc6 and deep rc6, and 7 would enable everything. "
> +		"default: -1 (use per-chip default)");
>  
>  int i915_enable_fbc __read_mostly = -1;
>  module_param_named(i915_enable_fbc, i915_enable_fbc, int, 0600);
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 554bef7..c17bccf 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -997,6 +997,27 @@ struct drm_i915_file_private {
>  
>  #include "i915_trace.h"
>  
> +/**
> + * RC6 is a special power stage which allows the GPU to enter an very
> + * low-voltage mode when idle, using down to 0V while at this stage.  This
> + * stage is entered automatically when the GPU is idle when RC6 support is
> + * enabled, and as soon as new workload arises GPU wakes up automatically as well.
> + *
> + * There are different RC6 modes available in Intel GPU, which differentiate
> + * among each other with the latency required to enter and leave RC6 and
> + * voltage consumed by the GPU in different states.
> + *
> + * The combination of the following flags define which states GPU is allowed
> + * to enter, while RC6 is the normal RC6 state, RC6p is the deep RC6, and
> + * RC6pp is deepest RC6. Their support by hardware varies according to the
> + * GPU, BIOS, chipset and platform. RC6 is usually the safest one and the one
> + * which brings the most power savings; deeper states save more power, but
> + * require higher latency to switch to and wake up.
> + */
> +#define INTEL_RC6_ENABLE			(1<<0)
> +#define INTEL_RC6p_ENABLE			(1<<1)
> +#define INTEL_RC6pp_ENABLE			(1<<2)
> +
>  extern struct drm_ioctl_desc i915_ioctls[];
>  extern int i915_max_ioctl;
>  extern unsigned int i915_fbpercrtc __always_unused;

Does it ever make sense to enable a deeper state without enabling the
one before it? Ie. would we ever enable RC6p and not RC6? Maybe we
don't want to have discrete bitfields and instead do:
> +#define INTEL_RC6_ENABLE			(1<<0)
> +#define INTEL_RC6p_ENABLE			(1<<1) | INTEL_RC6_ENABLE
> +#define INTEL_RC6pp_ENABLE			(1<<2) | INTEL_RC6p_ENABLE

Just a thought for the sake of hearing myself speak.