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.faine...@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;
  }