[PATCH v7 1/6] riscv: mm: dma-noncoherent: Switch using function pointers for cache management

[Biju Das]

Hi Christoph Hellwig,

> -----Original Message-----
> From: Christoph Hellwig <hch at infradead.org>
> Sent: Tuesday, April 4, 2023 6:29 AM
> To: Prabhakar <prabhakar.csengg at gmail.com>
> Cc: Arnd Bergmann <arnd at arndb.de>; Conor Dooley
> <conor.dooley at microchip.com>; Geert Uytterhoeven <geert+renesas at glider.be>;
> Heiko Stuebner <heiko at sntech.de>; Guo Ren <guoren at kernel.org>; Andrew Jones
> <ajones at ventanamicro.com>; Paul Walmsley <paul.walmsley at sifive.com>; Palmer
> Dabbelt <palmer at dabbelt.com>; Albert Ou <aou at eecs.berkeley.edu>; Samuel
> Holland <samuel at sholland.org>; linux-riscv at lists.infradead.org; Rob Herring
> <robh+dt at kernel.org>; Krzysztof Kozlowski
> <krzysztof.kozlowski+dt at linaro.org>; devicetree at vger.kernel.org; linux-
> kernel at vger.kernel.org; linux-renesas-soc at vger.kernel.org; Biju Das
> <biju.das.jz at bp.renesas.com>; Prabhakar Mahadev Lad <prabhakar.mahadev-
> lad.rj at bp.renesas.com>
> Subject: Re: [PATCH v7 1/6] riscv: mm: dma-noncoherent: Switch using
> function pointers for cache management
> 
> On Thu, Mar 30, 2023 at 09:42:12PM +0100, Prabhakar wrote:
> > From: Lad Prabhakar <prabhakar.mahadev-lad.rj at bp.renesas.com>
> >
> > Currently, selecting which CMOs to use on a given platform is done
> > using and ALTERNATIVE_X() macro. This was manageable when there were
> > just two CMO implementations, but now that there are more and more
> > platforms coming needing custom CMOs, the use of the ALTERNATIVE_X() macro
> is unmanageable.
> >
> > To avoid such issues this patch switches to use of function pointers
> > instead of ALTERNATIVE_X() macro for cache management (the only
> > drawback being performance over the previous approach).
> >
> > void (*clean_range)(unsigned long addr, unsigned long size); void
> > (*inv_range)(unsigned long addr, unsigned long size); void
> > (*flush_range)(unsigned long addr, unsigned long size);
> >
> 
> NAK.  Function pointers for somthing high performance as cache maintainance
> is a complete no-go.

Just a question, how does function pointer makes a performance difference compared to
ALTERNATIVE_X() macros?

On both cases, we are pushing function parameters to stack, jumping to the actual routine
And then on return pop the variables from stack. Am I missing something here?

Benchmark results by [1], shows that there is no performance degradation. I am not sure
What type of benchmarking used in this case and How accurate is this benchmark?

https://lore.kernel.org/linux-renesas-soc/40cdea465fef49a8a337b48e2a748138c66a08eb.camel@icenowy.me/T/#m093c1f3d8f7f0e15bd2909900bf137d5714c553c

Cheers,
Biju