Re: [PATCH v2] riscv: mm: still create swiotlb buffer for kmalloc() bouncing if required

[Jisheng Zhang <jszhang@kernel.org>]

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"

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.

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();