drm: Use a little stash on the stack to avoid kmalloc in most DRM ioctls.

The kmalloc was taking up about 1.5% of the CPU on an ioctl-heavy workload
(x11perf -aa10text on 965).  Initial results look like they have a
corresponding improvement in performance for aa10text, but more numbers might
not hurt.

Thanks to ajax for pointing out this performance regression I'd introduced
back in 2007.

[airlied: well I introduced it sneakily inside Eric's patch]

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 10 insertions, 5 deletions

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index c26ee0822a05..c4ada8b6295b 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -421,6 +421,7 @@ int drm_ioctl(struct inode *inode, struct file *filp,
         drm_ioctl_t *func;
         unsigned int nr = DRM_IOCTL_NR(cmd);
         int retcode = -EINVAL;
+        char stack_kdata[128];
         char *kdata = NULL;
 
         atomic_inc(&dev->ioctl_count);
@@ -459,10 +460,14 @@ int drm_ioctl(struct inode *inode, struct file *filp,
                 retcode = -EACCES;
         } else {
                 if (cmd & (IOC_IN | IOC_OUT)) {
-                        kdata = kmalloc(_IOC_SIZE(cmd), GFP_KERNEL);
-                        if (!kdata) {
-                                retcode = -ENOMEM;
-                                goto err_i1;
+                        if (_IOC_SIZE(cmd) <= sizeof(stack_kdata)) {
+                                kdata = stack_kdata;
+                        } else {
+                                kdata = kmalloc(_IOC_SIZE(cmd), GFP_KERNEL);
+                                if (!kdata) {
+                                        retcode = -ENOMEM;
+                                        goto err_i1;
+                                }
                         }
                 }
 
@@ -483,7 +488,7 @@ int drm_ioctl(struct inode *inode, struct file *filp,
         }
 
       err_i1:
-        if (kdata)
+        if (kdata != stack_kdata)
                 kfree(kdata);
         atomic_dec(&dev->ioctl_count);
         if (retcode)