__pv_phys_offset, inline assembly functions, and such like

[Russell King - ARM Linux]

On Fri, Mar 28, 2014 at 05:44:42PM +0100, Arnd Bergmann wrote:
> On Friday 28 March 2014 15:28:32 Russell King - ARM Linux wrote:
> > With the inline assembly eliminated from the macro, and __pv_phys_offset
> > converted to a PFN offset instead, the above assembly code shrinks
> > dramatically:
> > 
> >      a20:       e59331d4        ldr     r3, [r3, #468]  ; get_dma_ops
> >      a24:       e3001000        movw    r1, #0
> >                         a24: R_ARM_MOVW_ABS_NC  arm_dma_ops
> >      a28:       e3401000        movt    r1, #0		; &arm_dma_ops
> >                         a28: R_ARM_MOVT_ABS     arm_dma_ops
> >      a2c:       e3530000        cmp     r3, #0		; get_dma_ops
> >      a30:       01a03001        moveq   r3, r1		; get_dma_ops
> >      a34:       e28a1101        add     r1, sl, #1073741824 ; r1 = addr - PAGE_OFFSET
> >      a38:       e3002000        movw    r2, #0
> >                         a38: R_ARM_MOVW_ABS_NC  mem_map
> >      a3c:       e3402000        movt    r2, #0		; r2 = &mem_map
> >                         a3c: R_ARM_MOVT_ABS     mem_map
> >      a40:       e3a0e001        mov     lr, #1		; direction
> >      a44:       e1a01621        lsr     r1, r1, #12	; r1 = (addr - PAGE_OFFSET) >> 12 (pfn offset)
> >      a48:       e592c000        ldr     ip, [r2]	; ip = &mem_map[0]
> >      a4c:       e1a02a0a        lsl     r2, sl, #20	; r2 = part converted offset into page
> >      a50:       e58de000        str     lr, [sp]	; stack direction
> >      a54:       e3a0a000        mov     sl, #0		; attr
> >      a58:       e08c1281        add     r1, ip, r1, lsl #5 ; r1 = &mem_map[(addr - PAGE_OFFSET) >> 12]
> >      a5c:       e58da004        str     sl, [sp, #4]	; stack attr
> >      a60:       e1a02a22        lsr     r2, r2, #20	; r2 = offset
> >      a64:       e593c010        ldr     ip, [r3, #16]	; ops->map_page
> >      a68:       e1a03006        mov     r3, r6		; length
> >      a6c:       e12fff3c        blx     ip		; call map_page
> > 
> > With this in place, I see a reduction of 4220 bytes in the kernel image,
> > and that's from just fixing virt_to_page() and changing __pv_phys_offset
> > to be PFN-based (so it can be 32-bit in all cases.)
> 
> Ah, nice. Another idea: have you considered making the entire function extern?
> The only advantage I see in the inline version is saving one function call,
> but even with your new version there seems to be a noticeable size overhead.

The contribution above from virt_to_page() is just six instructions,
compared to 19 for the whole of dma_map_single().

So, there's much more advantage to killing the inline functions in
include/asm-generic/dma-mapping.h, and moving them out of line.  That's
where much of the above assembly is coming from.

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