Re: [PATCH] radeon: Shrink radeon_ring_write()

[Christian KÃnig]

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 KÃnig 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@xxxxxxxxxx>
> > > Cc: Christian KÃnig <christian.koenig@xxxxxxx>
> > > Cc: Alex Deucher <alexander.deucher@xxxxxxx>
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > ---
> > >    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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