kerneldoc polish will follow in the next patch.
Hopefully documenting the lru scan support a bit better spurse someone to give this a shot in the ttm eviction code. At least in i915 it helped quite a lot with memory thrashing on platforms where eviction was (we've fixed that too meanwhile) fairly expensive.
Signed-off-by: Daniel Vetter daniel.vetter@ffwll.ch --- Documentation/DocBook/drm.tmpl | 11 +++++++ drivers/gpu/drm/drm_mm.c | 67 ++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 78 insertions(+)
diff --git a/Documentation/DocBook/drm.tmpl b/Documentation/DocBook/drm.tmpl index ede383af1868..c6f916989b92 100644 --- a/Documentation/DocBook/drm.tmpl +++ b/Documentation/DocBook/drm.tmpl @@ -920,6 +920,17 @@ struct drm_gem_object * (*gem_prime_import)(struct drm_device *dev, <title>PRIME Function References</title> !Edrivers/gpu/drm/drm_prime.c </sect2> + <sect2> + <title>DRM MM Range Allocator</title> + <sect3> + <title>Overview</title> +!Pdrivers/gpu/drm/drm_mm.c Overview + </sect3> + <sect3> + <title>LRU Scan/Eviction Support</title> +!Pdrivers/gpu/drm/drm_mm.c lru scan roaster + </sect3> + </sect2> </sect1>
<!-- Internals: mode setting --> diff --git a/drivers/gpu/drm/drm_mm.c b/drivers/gpu/drm/drm_mm.c index d0a8e8482fe0..276a7a27c166 100644 --- a/drivers/gpu/drm/drm_mm.c +++ b/drivers/gpu/drm/drm_mm.c @@ -47,6 +47,45 @@ #include <linux/seq_file.h> #include <linux/export.h>
+/** + * DOC: Overview + * + * drm_mm provides a simple range allocator. The drivers are free to use the + * resource allocator from the linux core if it suits them, the upside of drm_mm + * is that it's in the DRM core. Which means that it's easier to extend for + * some of the crazier special purpose needs of gpus. + * + * The main data struct is &drm_mm, allocations are tracked in &drm_mm_node. + * Drivers are free to embed either of them into their own suitable + * datastructures. drm_mm itself will not do any allocations of its own, so if + * drivers choose not to embed nodes they need to still allocate them + * themselves. + * + * The range allocator also supports reservation of preallocated blocks. This is + * useful for taking over initial mode setting configurations from the firmware, + * where an object needs to be created which exactly matches the firmware's + * scanout target. As long as the range is still free it can be inserted anytime + * after the allocator is initialized, which helps with avoiding looped + * depencies in the driver load sequence. + * + * drm_mm maintains a stack of most recently freed holes, which of all + * simplistic datastructures seems to be a fairly decent approach to clustering + * allocations and avoiding too much fragmentation. This means free space + * searches are O(num_holes). Given that all the fancy features drm_mm supports + * something better would be fairly complex and since gfx thrashing is a fairly + * steep cliff not a real concern. Removing a node again is O(1). + * + * drm_mm supports a few features: Alignment and range restrictions can be + * supplied. Further more every &drm_mm_node has a color value (which is just an + * opaqua unsigned long) which in conjunction with a driver callback can be used + * to implement sophisticated placement restrictions. The i915 DRM driver uses + * this to implement guard pages between incompatible caching domains in the + * graphics TT. + * + * Finally iteration helpers to walk all nodes and all holes are provided as are + * some basic allocator dumpers for debugging. + */ + static struct drm_mm_node *drm_mm_search_free_generic(const struct drm_mm *mm, unsigned long size, unsigned alignment, @@ -400,6 +439,34 @@ void drm_mm_replace_node(struct drm_mm_node *old, struct drm_mm_node *new) EXPORT_SYMBOL(drm_mm_replace_node);
/** + * DOC: lru scan roaster + * + * Very often GPUs need to have continuous allocations for a given object. When + * evicting objects to make space for a new one it is therefore not most + * efficient when we simply start to select all objects from the tail of an LRU + * until there's a suitable hole: Especially for big objects or nodes that + * otherwise have special allocation constraints there's a good chance we evict + * lots of (smaller) objects unecessarily. + * + * The DRM range allocator supports this use-case through the scanning + * interfaces. First a scan operation needs to be initialized with + * drm_mm_init_scan() or drm_mm_init_scan_with_range(). The the driver adds + * objects to the roaster (probably by walking an LRU list, but this can be + * freely implemented) until a suitable hole is found or there's no further + * evitable object. + * + * The the driver must walk through all objects again in exactly the reverse + * order to restore the allocator state. Note that while the allocator is used + * in the scan mode no other operation is allowed. + * + * Finally the driver evicts all objects selected in the scan. Adding and + * removing an object is O(1), and since freeing a node is also O(1) the overall + * complexity is O(scanned_objects). So like the free stack which needs to be + * walked before a scan operation even begins this is linear in the number of + * objects. It doesn't seem to hurt badly. + */ + +/** * Initializa lru scanning. * * This simply sets up the scanning routines with the parameters for the desired