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SubjectRe: [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB
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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@gmail.com>
>
> 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;
>> }
>>
>>
>

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