Re: [PATCH] radeon: Shrink radeon_ring_write()

[Christian König]

Well thinking about this we could actually get completely ride of 
decrementing count_dw if we precalculate the last valid wptr instead.

That would also reduce the number of error messages to once every ring 
submit, but I would say this is actually a rather positive side effect.

Regards,
Christian.

On 18.05.2015 20:25, Denys Vlasenko wrote:
On 05/18/2015 08:11 PM, Christian König wrote:
De-duplicating the error message is probably a good idea,
but are the remaining code changes really necessary for the size reduction?
The conversion from

    if (ring->count_dw <= 0)
         error;
    ...
    ring->count_dw--;

to

    if (--ring->count_dw < 0)
         error;
    ...

eliminates one access to ring->count_dw.


Conversion from

     ring->ring[ring->wptr++] = v;
     ring->wptr &= ring->ptr_mask;

to

     ring->ring[ring->wptr] = v;
     ring->wptr = (ring->wptr + 1) & ring->ptr_mask;

ideally (with infinitely smart compiler)
shouldn't be necessary, but at least for my gcc
version it eliminates one additional insn:
gcc copies ring->wptr to a scratch reg, then increments it,
then uses scratch reg for addressing...


On 18.05.2015 19:59, Denys Vlasenko wrote:
Inlined radeon_ring_write() has 729 callers, which amounts to about 50000
bytes of code. however, deinlining it is probably too much
of a performance impact.

This patch shrinks slow path a bit and optimizes fast path.
Comparison of generated machine code is below:

old___________________________________ new____________________________
55             push %rbp               55         push %rbp
4889e5         mov  %rsp,%rbp          ff4f38     decl 0x38(%rdi)
4154           push %r12               4889e5     mov  %rsp,%rbp
4189f4         mov  %esi,%r12d         4154       push %r12
53             push %rbx               4189f4     mov  %esi,%r12d
837f3800       cmpl $0x0,0x38(%rdi)    53         push %rbx
4889fb         mov  %rdi,%rbx          4889fb     mov  %rdi,%rbx
7f0e           jg   <.Lbl>             7905       jns  <.Lbl>
48c7c78f51a785 mov  $message,%rdi
31c0           xor  %eax,%eax
e89306f9ff     call <drm_err>          e8cbffffff call <radeon_ring_overflow>
      .Lbl:
8b4328         mov  0x28(%rbx),%eax    8b5328     mov  0x28(%rbx),%edx
488b5308       mov  0x8(%rbx),%rdx     488b4308   mov  0x8(%rbx),%rax
89c1           mov  %eax,%ecx          488d0490   lea  (%rax,%rdx,4),%rax
ffc0           inc  %eax
488d148a       lea  (%rdx,%rcx,4),%rdx 448920     mov  %r12d,(%rax)
448922         mov  %r12d,(%rdx)       8b4328     mov  0x28(%rbx),%eax
234354         and  0x54(%rbx),%eax    ff4b34     decl 0x34(%rbx)
ff4b38         decl 0x38(%rbx)         ffc0       inc  %eax
ff4b34         decl 0x34(%rbx)         234354     and  0x54(%rbx),%eax
894328         mov  %eax,0x28(%rbx)    894328     mov  %eax,0x28(%rbx)
5b             pop  %rbx               5b         pop  %rbx
415c           pop  %r12               415c       pop  %r12
5d             pop  %rbp               5d         pop  %rbp

This shaves off more than 10 kbytes of code off the kernel:

      text     data      bss       dec     hex filename
85657104 22294872 20627456 128579432 7a9f768 vmlinux.before
85646544 22294872 20627456 128568872 7a9ce28 vmlinux

Signed-off-by: Denys Vlasenko <dvlas...@redhat.com>
Cc: Christian König <christian.koe...@amd.com>
Cc: Alex Deucher <alexander.deuc...@amd.com>
Cc: linux-kernel@vger.kernel.org
---
   drivers/gpu/drm/radeon/radeon.h      | 11 +++++------
   drivers/gpu/drm/radeon/radeon_ring.c |  5 +++++
   2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/radeon/radeon.h b/drivers/gpu/drm/radeon/radeon.h
index bb6b25c..9106873 100644
--- a/drivers/gpu/drm/radeon/radeon.h
+++ b/drivers/gpu/drm/radeon/radeon.h
@@ -2658,14 +2658,13 @@ void radeon_atombios_fini(struct radeon_device *rdev);
    *
    * Write a value to the requested ring buffer (all asics).
    */
+void radeon_ring_overflow(void);
   static inline void radeon_ring_write(struct radeon_ring *ring, uint32_t v)
   {
-    if (ring->count_dw <= 0)
-        DRM_ERROR("radeon: writing more dwords to the ring than expected!\n");
-
-    ring->ring[ring->wptr++] = v;
-    ring->wptr &= ring->ptr_mask;
-    ring->count_dw--;
+    if (--ring->count_dw < 0)
+        radeon_ring_overflow();
+    ring->ring[ring->wptr] = v;
+    ring->wptr = (ring->wptr + 1) & ring->ptr_mask;
       ring->ring_free_dw--;
   }
   diff --git a/drivers/gpu/drm/radeon/radeon_ring.c 
b/drivers/gpu/drm/radeon/radeon_ring.c
index 2456f69..8204c23 100644
--- a/drivers/gpu/drm/radeon/radeon_ring.c
+++ b/drivers/gpu/drm/radeon/radeon_ring.c
@@ -126,6 +126,11 @@ int radeon_ring_alloc(struct radeon_device *rdev, struct 
radeon_ring *ring, unsi
       return 0;
   }
   +void radeon_ring_overflow(void)
+{
+    DRM_ERROR("radeon: writing more dwords to the ring than expected!\n");
+}
+
   /**
    * radeon_ring_lock - lock the ring and allocate space on it
    *

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