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Message-ID: <555A3720.80600@amd.com>
Date: Mon, 18 May 2015 21:01:52 +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()
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 <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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