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

Ben Widawsky
2013-07-11T05:03:34Z

On Wed, Jul 10, 2013 at 05:16:54PM -0700, Chad Versace wrote:
> 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?

Sounds like Chris wants the same anyway, but no. Getparam is always an
int. Just the name is, 'has'

>
> >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 ;)
>

Still arguing over whether or not to try to pull it from the x86 core. I
tried to get some comment from hpa today , but haven't heard anything
yet.

I do agree if we can just take the code from the x86 core it would be
best, but AFAICT I'd need to change some code, or do an almost equal
amount of logic to determine l2 vs. l3. Thinking about it now though, I
suppose I can assume if HAS_LLC, then whatever cache size was obtained
during boot is l3... dunno.

--
Ben Widawsky, Intel Open Source Technology Center
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