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    SubjectRe: [RFC PATCH v6 1/6] riscv: mm: dma-noncoherent: Switch using function pointers for cache management
    On Sat, Jan 7, 2023, at 00:29, Conor Dooley wrote:
    > On Fri, Jan 06, 2023 at 11:31:33PM +0100, Arnd Bergmann wrote:
    >> On Fri, Jan 6, 2023, at 19:55, Prabhakar wrote:
    >> > From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
    >> > +struct riscv_cache_ops zicbom_cmo_ops = {
    >> > + .clean_range = &zicbom_cmo_clean_range,
    >> > + .inv_range = &zicbom_cmo_inval_range,
    >> > + .flush_range = &zicbom_cmo_flush_range,
    >> > +};
    >> > +#else
    >> > +struct riscv_cache_ops zicbom_cmo_ops = {
    >> > + .clean_range = NULL,
    >> > + .inv_range = NULL,
    >> > + .flush_range = NULL,
    >> > + .riscv_dma_noncoherent_cmo_ops = NULL,
    >> > +};
    >> > +#endif
    >> > +EXPORT_SYMBOL(zicbom_cmo_ops);
    >>
    >> Same here: If the ZICBOM ISA is disabled, nothing should
    >> reference zicbom_cmo_ops.
    >
    >> Also, since ZICBOM is a standard
    >> extension, I think it makes sense to always have it enabled,
    >> at least whenever noncoherent DMA is supported, that way
    >> it can be the default that gets used in the absence of any
    >> nonstandard cache controller.
    >
    > While I think of it, this is not possible as Zicbom requires toolchain
    > support whereas the alternative methods for non-coherent DMA do not.

    Ah, I see. Would it be possible to use the same .long trick
    as in the other ones though? Something like

    #if CONFIG_AS_VERSION >= 23600 /* or whichever version */
    /* proper inline asm */
    #else
    /* .long hack */
    #endif

    That way everyone can use it, and the hack would automatically
    go away in a few years after linux requires a newer toolchain.

    Alternatively, the entire noncoherent-dma support could be
    made to depend on whichever toolchain introduced Zicbom.

    Arnd

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