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

[Ben Dooks]

On 08/03/2023 14:48, Ben Dooks wrote:
> On 08/03/2023 13:19, heiko at sntech.de wrote:
>> Am Mittwoch, 8. März 2023, 10:35:14 CET schrieb Ben Dooks:
>>> 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.
>>
>> CONFIG_RISCV_ISA_ZICBOM does a
>>     select RISCV_DMA_NONCOHERENT
>> same as CONFIG_ERRATA_THEAD_CMO
>>
>> So dma-noncoherent gets build if you enable at least one of them.
>> What am I missing?
> 
> Ah, I think (and Connor also pointed out) that currently the config
> is selceted from either user of the dma-noncoherent.c code. I think
> thereore we can probably keep this as is

After a discussion with Arnd it looks like I was using the DMA api
wrong and was after dma_alloc_noncoherent or similar.

These patches can be dropped

-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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