RE: [RFC 1/1] swiotlb: Reduce calls to swiotlb_find_pool()

[Michael Kelley <mhklinux@xxxxxxxxxxx>]

From: Petr Tesařík <petr@xxxxxxxxxxx> Sent: Thursday, June 27, 2024 12:21 AM

[...]

> > @@ -187,10 +169,13 @@ static inline bool is_swiotlb_buffer(struct device 
> > *dev, phys_addr_t paddr)
> >      * This barrier pairs with smp_mb() in swiotlb_find_slots().
> >      */
> >     smp_rmb();
> > -   return READ_ONCE(dev->dma_uses_io_tlb) &&
> > -           swiotlb_find_pool(dev, paddr);
> > +   if (READ_ONCE(dev->dma_uses_io_tlb))
> > +           return swiotlb_find_pool(dev, paddr);
> > +   return NULL;
> >  #else
> > -   return paddr >= mem->defpool.start && paddr < mem->defpool.end;
> > +   if (paddr >= mem->defpool.start && paddr < mem->defpool.end)
> > +           return &mem->defpool;
> 
> Why are we open-coding swiotlb_find_pool() here? It does not make a
> difference now, but if swiotlb_find_pool() were to change, both places
> would have to be updated.
> 
> Does it save a reload from dev->dma_io_tlb_mem? IOW is the compiler
> unable to optimize it away?
> 
> What about this (functionally identical) variant:
> 
> #ifdef CONFIG_SWIOTLB_DYNAMIC
>       smp_rmb();
>       if (!READ_ONCE(dev->dma_uses_io_tlb))
>               return NULL;
> #else
>       if (paddr < mem->defpool.start || paddr >= mem->defpool.end);
>               return NULL;
> #endif
> 
>       return swiotlb_find_pool(dev, paddr);
> 

Yeah, I see your point. I'll try this and see what the generated code
looks like. It might take me a couple of days to get to it.

Michael