linux-kernel - Re: [PATCH] radeon: Shrink radeon_ring

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Date:	Mon, 18 May 2015 20:11:26 +0200
From:	Christian König <christian.koenig@....com>
To:	Denys Vlasenko <dvlasenk@...hat.com>
CC:	Alex Deucher <alexander.deucher@....com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] radeon: Shrink radeon_ring_write()

De-duplicating the error message is probably a good idea, but are the 
remaining code changes really necessary for the size reduction?

Regards,
Christian.

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 <dvlasenk@...hat.com>
> Cc: Christian König <christian.koenig@....com>
> Cc: Alex Deucher <alexander.deucher@....com>
> Cc: linux-kernel@...r.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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