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Message-ID: <b7c1d204-22b0-4a04-a4bb-1aaf4e643c22@ghiti.fr>
Date: Tue, 16 Jan 2024 10:07:16 +0100
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Subject: Re: [PATCH v2] riscv: mm: still create swiotlb buffer for kmalloc()
 bouncing if required
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To: Jisheng Zhang <jszhang@kernel.org>
Cc: Paul Walmsley <paul.walmsley@sifive.com>,
 Palmer Dabbelt <palmer@dabbelt.com>, Albert Ou <aou@eecs.berkeley.edu>,
 linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
References: <20231202134224.4029-1-jszhang@kernel.org>
 <aea8222e-ec0a-4844-abd0-7fe102bc2bec@ghiti.fr> <ZaZCqAir6BdiN80K@xhacker>
From: Alexandre Ghiti <alex@ghiti.fr>
In-Reply-To: <ZaZCqAir6BdiN80K@xhacker>
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On 16/01/2024 09:47, Jisheng Zhang wrote:
> On Tue, Jan 16, 2024 at 09:23:47AM +0100, Alexandre Ghiti wrote:
>> Hi Jisheng,
>>
>> On 02/12/2023 14:42, Jisheng Zhang wrote:
>>> After commit f51f7a0fc2f4 ("riscv: enable DMA_BOUNCE_UNALIGNED_KMALLOC
>>> for !dma_coherent"), for non-coherent platforms with less than 4GB
>>> memory, we rely on users to pass "swiotlb=mmnn,force" kernel parameters
>>> to enable DMA bouncing for unaligned kmalloc() buffers. Now let's go
>>> further: If no bouncing needed for ZONE_DMA, let kernel automatically
>>> allocate 1MB swiotlb buffer per 1GB of RAM for kmalloc() bouncing on
>>> non-coherent platforms, so that no need to pass "swiotlb=mmnn,force"
>>> any more.
>> IIUC, DMA_BOUNCE_UNALIGNED_KMALLOC is enabled for all non-coherent
>> platforms, even those with less than 4GB of memory. But the DMA bouncing
>> (which is necessary to enable kmalloc-8/16/32/96...) was not enabled unless
>> the user specified "swiotlb=mmnn,force" on the kernel command line. But does
>> that mean that if the user did not specify "swiotlb=mmnn,force", the
>> kmalloc-8/16/32/96 were enabled anyway and the behaviour was wrong (by lack
>> of DMA bouncing)?
> Hi Alex,
>
> For coherent platforms, kmalloc-8/16/32/96 was enabled.
>
> For non-coherent platforms, if memory is more than 4GB, kmalloc-8/16/32/96 was enabled.
>
> For non-coherent platforms, if memory is less than 4GB, kmalloc-8/16/32/96 was not
> enabled. If users want kmalloc-8/16/32/96, we rely on users to pass "swiotlb=mmnn,force"


That's what I was unsure of :)


>
> This patch tries to remove the "swiotlb=mmnn,force" requirement for the
> last case. After this patch, kernel automatically uses "1MB swiotlb buffer per
> 1GB of RAM for kmalloc() bouncing" by default.
>
> So this is an enhancement.


Great, so you can add:

Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>

Thanks,

Alex


>
> Thanks
>> I'm trying to understand if that's a fix or an enhancement.
>>
>> Thanks,
>>
>> Alex
>>
>>
>>> The math of "1MB swiotlb buffer per 1GB of RAM for kmalloc() bouncing"
>>> is taken from arm64. Users can still force smaller swiotlb buffer by
>>> passing "swiotlb=mmnn".
>>>
>>> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
>>> ---
>>>
>>> since v2:
>>>    - fix build error if CONFIG_RISCV_DMA_NONCOHERENT=n
>>>
>>>    arch/riscv/include/asm/cache.h |  2 +-
>>>    arch/riscv/mm/init.c           | 16 +++++++++++++++-
>>>    2 files changed, 16 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/riscv/include/asm/cache.h b/arch/riscv/include/asm/cache.h
>>> index 2174fe7bac9a..570e9d8acad1 100644
>>> --- a/arch/riscv/include/asm/cache.h
>>> +++ b/arch/riscv/include/asm/cache.h
>>> @@ -26,8 +26,8 @@
>>>    #ifndef __ASSEMBLY__
>>> -#ifdef CONFIG_RISCV_DMA_NONCOHERENT
>>>    extern int dma_cache_alignment;
>>> +#ifdef CONFIG_RISCV_DMA_NONCOHERENT
>>>    #define dma_get_cache_alignment dma_get_cache_alignment
>>>    static inline int dma_get_cache_alignment(void)
>>>    {
>>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>>> index 2e011cbddf3a..cbcb9918f721 100644
>>> --- a/arch/riscv/mm/init.c
>>> +++ b/arch/riscv/mm/init.c
>>> @@ -162,11 +162,25 @@ static void print_vm_layout(void) { }
>>>    void __init mem_init(void)
>>>    {
>>> +	bool swiotlb = max_pfn > PFN_DOWN(dma32_phys_limit);
>>>    #ifdef CONFIG_FLATMEM
>>>    	BUG_ON(!mem_map);
>>>    #endif /* CONFIG_FLATMEM */
>>> -	swiotlb_init(max_pfn > PFN_DOWN(dma32_phys_limit), SWIOTLB_VERBOSE);
>>> +	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && !swiotlb &&
>>> +	    dma_cache_alignment != 1) {
>>> +		/*
>>> +		 * If no bouncing needed for ZONE_DMA, allocate 1MB swiotlb
>>> +		 * buffer per 1GB of RAM for kmalloc() bouncing on
>>> +		 * non-coherent platforms.
>>> +		 */
>>> +		unsigned long size =
>>> +			DIV_ROUND_UP(memblock_phys_mem_size(), 1024);
>>> +		swiotlb_adjust_size(min(swiotlb_size_or_default(), size));
>>> +		swiotlb = true;
>>> +	}
>>> +
>>> +	swiotlb_init(swiotlb, SWIOTLB_VERBOSE);
>>>    	memblock_free_all();
>>>    	print_vm_layout();