Hi Heiko, On Fri, Nov 4, 2022 at 11:07 PM Heiko Stübner <heiko@xxxxxxxxx> wrote: > > Hi Prabhakar, > > Am Freitag, 4. November 2022, 23:09:15 CET schrieb Lad, Prabhakar: > > When trying to add non coherent DMA support for the RZ/Five SoC, I was > > suggested to use the ALTERNATIVE() macro [0]. > > > > My initial intention was to implement the alternative_call_x() > > something like what x86 has and from there call the C function, So > > while doing some experiments I realized that using the asm "call > > <func>" would cause adding the whole lot of registers in the clobbers > > list. > > > > As per my understanding the only correct approach to > > alternative_call_x() is if the function itself is inline assembly then > > we know the exact registers being used and then we could add them > > appropriately in the clobbers list. > > I'm working in a quite similar area right now and I've Cc'ed you on a > RFC series when I'm also worked on makeing call work in alternatives. > Great!. I'll go through the series and take some inspiration from it. > As you noted, using assembler for the call'ed function makes things > way easier to handle. > :-) > > The problem with the above approach being we have quite a lot of C > > code [1] for the CMO and it also requires some register writes of the > > l2 cache controller, converting this to assembly would be a pain. > > > > * Is my understanding correct? > > * Is there any other approach I am missing? > > * Is there no way out apart from implementing inline asm? > > In the inline-asm part that will form the alternative, can you > not just save and restore the registers manually around the > actual call? > > ALTERNATIVE( > [1] ... nops ..., > [2] ... CMO ..., > [3] ... THEAD ..., > [4] save registers to stack > call ax45mp dma-func > restore registers from stack > > That way you would no incur many penalties to the other alternatives > I think. > Agreed, thanks for the suggestion. I'll continue my investigation in this direction. Cheers, Prabhakar
