Hi Christoph Hellwig, > -----Original Message----- > From: Christoph Hellwig <hch@xxxxxxxxxxxxx> > Sent: Tuesday, April 4, 2023 6:29 AM > To: Prabhakar <prabhakar.csengg@xxxxxxxxx> > Cc: Arnd Bergmann <arnd@xxxxxxxx>; Conor Dooley > <conor.dooley@xxxxxxxxxxxxx>; Geert Uytterhoeven <geert+renesas@xxxxxxxxx>; > Heiko Stuebner <heiko@xxxxxxxxx>; Guo Ren <guoren@xxxxxxxxxx>; Andrew Jones > <ajones@xxxxxxxxxxxxxxxx>; Paul Walmsley <paul.walmsley@xxxxxxxxxx>; Palmer > Dabbelt <palmer@xxxxxxxxxxx>; Albert Ou <aou@xxxxxxxxxxxxxxxxx>; Samuel > Holland <samuel@xxxxxxxxxxxx>; linux-riscv@xxxxxxxxxxxxxxxxxxx; Rob Herring > <robh+dt@xxxxxxxxxx>; Krzysztof Kozlowski > <krzysztof.kozlowski+dt@xxxxxxxxxx>; devicetree@xxxxxxxxxxxxxxx; linux- > kernel@xxxxxxxxxxxxxxx; linux-renesas-soc@xxxxxxxxxxxxxxx; Biju Das > <biju.das.jz@xxxxxxxxxxxxxx>; Prabhakar Mahadev Lad <prabhakar.mahadev- > lad.rj@xxxxxxxxxxxxxx> > 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@xxxxxxxxxxxxxx> > > > > 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@xxxxxxxxxx/T/#m093c1f3d8f7f0e15bd2909900bf137d5714c553c Cheers, Biju
