Re: [RFC PATCH v6 1/6] riscv: mm: dma-noncoherent: Switch using function pointers for cache management

[Arnd Bergmann]

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@xxxxxxxxxxxxxx>
>> > +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