[PATCH 2/2] riscv: mm: add pgprot_dmacoherent for zicbom

[Conor Dooley]

On Wed, Mar 08, 2023 at 09:35:14AM +0000, Ben Dooks wrote:
> On 07/03/2023 20:58, Ben Dooks wrote:
> > If the system uses both ZICBOM and SVPBMT then currently SVPBMT will be
> > used for DMA allocated memory even though ZICBOM gives us the cache ops
> > to use cached memory and clean/flush them as needed by the DMA code.
> > 
> > Fix this by adding pgprot_dmacoherent() which is used by the allocator
> > code to map the dma memory, thus allowing the return of suitably mapped
> > memory for any use of dma_alloc_attrs() code. This s uses the added
> > riscv_page_dmacoherent() which will work out the correct page flags to
> > return using ALT_SVPBMT_ZICBOM() to runtime patch the right result.
> > 
> > Note, we can't just disable SVPBMT as it will be neede for things like
> > ioremap() which don't have assoicated cache management operations.
> > 
> > Signed-off-by: Ben Dooks <ben.dooks at codethink.co.uk>
> > ---
> >   arch/riscv/include/asm/errata_list.h | 16 ++++++++++++++++
> >   arch/riscv/include/asm/pgtable-64.h  | 10 ++++++++++
> >   2 files changed, 26 insertions(+)
> > 
> > diff --git a/arch/riscv/include/asm/errata_list.h b/arch/riscv/include/asm/errata_list.h
> > index fb1a810f3d8c..49ed2e7984a7 100644
> > --- a/arch/riscv/include/asm/errata_list.h
> > +++ b/arch/riscv/include/asm/errata_list.h
> > @@ -62,6 +62,22 @@ asm(ALTERNATIVE_2("li %0, 0\t\nnop",					\
> >   		  "I"(ALT_SVPBMT_SHIFT),				\
> >   		  "I"(ALT_THEAD_PBMT_SHIFT))
> > +#define ALT_SVPBMT_ZICBOM(_val, prot)					\
> > +asm(ALTERNATIVE_3("li %0, 0\t\nnop",					\
> > +		  "li %0, %1\t\nslli %0,%0,%3", 0,			\
> > +			RISCV_ISA_EXT_SVPBMT, CONFIG_RISCV_ISA_SVPBMT,	\
> > +		  "li %0, 0\t\nnop", 0,					\
> > +			RISCV_ISA_EXT_ZICBOM, CONFIG_RISCV_ISA_ZICBOM,	\
> 
> so, I tink this needs to be selected on CONFIG_RISCV_DMA_NONCOHERENT
> as just having ZICBOM in the ISA isn't enough to actually use it, you'll
> need the dma-noncoherent.o being built to do the cache management.

I think we are okay here w/ ZICBOM, as it selects RISCV_DMA_NONCOHERENT:
config RISCV_ISA_ZICBOM
	bool "Zicbom extension support for non-coherent DMA operation"
	depends on !XIP_KERNEL && MMU
	default y
	select RISCV_ALTERNATIVE
	select RISCV_DMA_NONCOHERENT

In fact, I think that RISCV_DMA_NONCOHERENT would not be correct here,
as the THEAD CMO erratum selects RISCV_DMA_NONCOHERENT, so its presence
doesn't mean that the kernel supports Zicbom.

> > +		  "li %0, %2\t\nslli %0,%0,%4", THEAD_VENDOR_ID,	\
> > +			ERRATA_THEAD_PBMT, CONFIG_ERRATA_THEAD_PBMT)	\
> > +		: "=r"(_val)						\
> > +		: "I"(prot##_SVPBMT >> ALT_SVPBMT_SHIFT),		\
> > +		  "I"(prot##_THEAD >> ALT_THEAD_PBMT_SHIFT),		\
> > +		  "I"(ALT_SVPBMT_SHIFT),				\
> > +		  "I"(ALT_THEAD_PBMT_SHIFT))
> > +
> > +
> > +
> >   #ifdef CONFIG_ERRATA_THEAD_PBMT
> >   /*
> >    * IO/NOCACHE memory types are handled together with svpbmt,
> > diff --git a/arch/riscv/include/asm/pgtable-64.h b/arch/riscv/include/asm/pgtable-64.h
> > index 42a042c0e13e..e0d2e5fda5a4 100644
> > --- a/arch/riscv/include/asm/pgtable-64.h
> > +++ b/arch/riscv/include/asm/pgtable-64.h
> > @@ -126,10 +126,20 @@ static inline u64 riscv_page_io(void)
> >   	return val;
> >   }
> > +static inline u64 riscv_page_dmacoherent(void)
> > +{
> > +	u64 val;
> > +
> > +	ALT_SVPBMT_ZICBOM(val, _PAGE_IO);
> > +	return val;
> > +}
> > +
> >   #define _PAGE_NOCACHE		riscv_page_nocache()
> >   #define _PAGE_IO		riscv_page_io()
> >   #define _PAGE_MTMASK		riscv_page_mtmask()
> > +#define pgprot_dmacoherent(__prot) __pgprot(pgprot_val(__prot) | riscv_page_dmacoherent())
> > +
> >   /* Set of bits to preserve across pte_modify() */
> >   #define _PAGE_CHG_MASK  (~(unsigned long)(_PAGE_PRESENT | _PAGE_READ |	\
> >   					  _PAGE_WRITE | _PAGE_EXEC |	\
> 
> -- 
> Ben Dooks				http://www.codethink.co.uk/
> Senior Engineer				Codethink - Providing Genius
> 
> https://www.codethink.co.uk/privacy.html
> 
> 
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