Re: [PATCH] drm/i915: Expose LLC size to user space

Chad Versace
2013-07-11T00:16:58Z

On 07/09/2013 07:58 PM, Ben Widawsky wrote:
> The algorithm/information was originally written by Chad, though I
> changed the control flow, and I think his original code had a couple of
> bugs, though I didn't look very hard before rewriting. That could have
> also been different interpretations of the spec.
>
> The excellent comments remain entirely copied from Chad's code.
>
> I've tested this on two platforms, and it seems to perform how I want.
>
> CC: Chad Versace <chad.versace@linux.intel.com>
> CC: Bryan Bell <bryan.j.bell@intel.com>
> Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
> ---
> drivers/gpu/drm/i915/i915_dma.c | 2 +-
> drivers/gpu/drm/i915/i915_drv.h | 2 ++
> drivers/gpu/drm/i915/i915_gem.c | 53 +++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 56 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c
> index 0e22142..377949e 100644
> --- a/drivers/gpu/drm/i915/i915_dma.c
> +++ b/drivers/gpu/drm/i915/i915_dma.c
> @@ -974,7 +974,7 @@ static int i915_getparam(struct drm_device *dev, void *data,
> value = 1;
> break;
> case I915_PARAM_HAS_LLC:
> - value = HAS_LLC(dev);
> + value = dev_priv->llc_size;
> break;
> case I915_PARAM_HAS_ALIASING_PPGTT:
> value = dev_priv->mm.aliasing_ppgtt ? 1 : 0;

I would like to see a dedicated param for the llc size. 'has' is always
boolean valued, right?

> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index c8d6104..43a549d 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -1187,6 +1187,8 @@ typedef struct drm_i915_private {
> /* Old dri1 support infrastructure, beware the dragons ya fools entering
> * here! */
> struct i915_dri1_state dri1;
> +
> + size_t llc_size;
> } drm_i915_private_t;
>
> /* Iterate over initialised rings */
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index af61be8..a070686 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -4282,6 +4282,57 @@ i915_gem_lastclose(struct drm_device *dev)
> DRM_ERROR("failed to idle hardware: %d\n", ret);
> }
>
> +/**
> + * Return the size, in bytes, of the CPU L3 cache size. If the CPU has no L3
> + * cache, or if an error occurs in obtaining the cache size, then return 0.
> + * From "Intel Processor Identification and the CPUID Instruction > 5.15
> + * Deterministic Cache Parmaeters (Function 04h)":
> + * When EAX is initialized to a value of 4, the CPUID instruction returns
> + * deterministic cache information in the EAX, EBX, ECX and EDX registers.
> + * This function requires ECX be initialized with an index which indicates
> + * which cache to return information about. The OS is expected to call this
> + * function (CPUID.4) with ECX = 0, 1, 2, until EAX[4:0] == 0, indicating no
> + * more caches. The order in which the caches are returned is not specified
> + * and may change at Intel's discretion.
> + *
> + * Equation 5-4. Calculating the Cache Size in bytes:
> + * = (Ways +1) � (Partitions +1) � (Line Size +1) � (Sets +1)
> + * = (EBX[31:22] +1) � (EBX[21:12] +1) � (EBX[11:0] +1 � (ECX + 1)
> + */
> +static size_t get_llc_size(struct drm_device *dev)
> +{
> + u8 cnt = 0;
> + unsigned int eax, ebx, ecx, edx;
> +
> + if (!HAS_LLC(dev))
> + return 0;
> +
> + do {
> + uint32_t cache_level;
> + uint32_t associativity, line_partitions, line_size, sets;
> +
> + eax = 4;
> + ecx = cnt;
> + __cpuid(&eax, &ebx, &ecx, &edx);
> +
> + cache_level = (eax >> 5) & 0x7;
> + if (cache_level != 3)
> + continue;
> +
> + associativity = ((ebx >> 22) & 0x3ff) + 1;
> + line_partitions = ((ebx >> 12) & 0x3ff) + 1;
> + line_size = (ebx & 0xfff) + 1;
> + sets = ecx + 1;
> +
> + return associativity * line_partitions * line_size * sets;
> + } while (eax & 0x1f && ++cnt);
> +
> + /* Let user space know we have LLC, but we can't figure it out */
> + DRM_DEBUG_DRIVER("Couldn't find LLC size. Bug?\n");
> + return 1;
> +}

This function looks good to me, since I wrote most of it ;)

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