Re: [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB

[Robin Murphy]

On 2021-03-18 19:22, Florian Fainelli wrote:
> On 3/18/2021 12:18 PM, Florian Fainelli wrote:
> > It may be useful to disable the SWIOTLB completely for testing or when a
> > platform is known not to have any DRAM addressing limitations what so
> > ever.

Isn't that what "swiotlb=noforce" is for? If you're confident that we've 
really ironed out *all* the awkward corners that used to blow up if 
various internal bits were left uninitialised, then it would make sense 
to just tweak the implementation of what we already have.

I wouldn't necessarily disagree with adding "off" as an additional alias 
for "noforce", though, since it does come across as a bit wacky for 
general use.

> > Signed-off-by: Florian Fainelli <f.fainelli@xxxxxxxxx>
>
> Christoph, in addition to this change, how would you feel if we
> qualified the swiotlb_init() in arch/arm/mm/init.c with a:
>
>
> if (memblock_end_of_DRAM() >= SZ_4G)
> 	swiotlb_init(1)

Modulo "swiotlb=force", of course ;)

Robin.

> right now this is made unconditional whenever ARM_LPAE is enabled which
> is the case for the platforms I maintain (ARCH_BRCMSTB) however we do
> not really need a SWIOTLB so long as the largest DRAM physical address
> does not exceed 4GB AFAICT.
>
> Thanks!
>
> > ---
> >   Documentation/admin-guide/kernel-parameters.txt | 1 +
> >   include/linux/swiotlb.h                         | 1 +
> >   kernel/dma/swiotlb.c                            | 9 +++++++++
> >   3 files changed, 11 insertions(+)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 04545725f187..b0223e48921e 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -5278,6 +5278,7 @@
> >   			force -- force using of bounce buffers even if they
> >   			         wouldn't be automatically used by the kernel
> >   			noforce -- Never use bounce buffers (for debugging)
> > +			off -- Completely disable SWIOTLB
> >   
> >   	switches=	[HW,M68k]
> >   
> > diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> > index 5857a937c637..23f86243defe 100644
> > --- a/include/linux/swiotlb.h
> > +++ b/include/linux/swiotlb.h
> > @@ -15,6 +15,7 @@ enum swiotlb_force {
> >   	SWIOTLB_NORMAL,		/* Default - depending on HW DMA mask etc. */
> >   	SWIOTLB_FORCE,		/* swiotlb=force */
> >   	SWIOTLB_NO_FORCE,	/* swiotlb=noforce */
> > +	SWIOTLB_OFF,		/* swiotlb=off */
> >   };
> >   
> >   /*
> > diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> > index c10e855a03bc..d7a4a789c7d3 100644
> > --- a/kernel/dma/swiotlb.c
> > +++ b/kernel/dma/swiotlb.c
> > @@ -126,6 +126,8 @@ setup_io_tlb_npages(char *str)
> >   	} else if (!strcmp(str, "noforce")) {
> >   		swiotlb_force = SWIOTLB_NO_FORCE;
> >   		io_tlb_nslabs = 1;
> > +	} else if (!strcmp(str, "off")) {
> > +		swiotlb_force = SWIOTLB_OFF;
> >   	}
> >   
> >   	return 0;
> > @@ -229,6 +231,9 @@ int __init swiotlb_init_with_tbl(char *tlb, unsigned long nslabs, int verbose)
> >   	unsigned long i, bytes;
> >   	size_t alloc_size;
> >   
> > +	if (swiotlb_force == SWIOTLB_OFF)
> > +		return 0;
> > +
> >   	bytes = nslabs << IO_TLB_SHIFT;
> >   
> >   	io_tlb_nslabs = nslabs;
> > @@ -284,6 +289,9 @@ swiotlb_init(int verbose)
> >   	unsigned char *vstart;
> >   	unsigned long bytes;
> >   
> > +	if (swiotlb_force == SWIOTLB_OFF)
> > +		goto out;
> > +
> >   	if (!io_tlb_nslabs) {
> >   		io_tlb_nslabs = (default_size >> IO_TLB_SHIFT);
> >   		io_tlb_nslabs = ALIGN(io_tlb_nslabs, IO_TLB_SEGSIZE);
> > @@ -302,6 +310,7 @@ swiotlb_init(int verbose)
> >   		io_tlb_start = 0;
> >   	}
> >   	pr_warn("Cannot allocate buffer");
> > +out:
> >   	no_iotlb_memory = true;
> >   }
> >