Re: [PATCH] mmc: sdhci-xenon: Fix 2G limitation on AC5 SoC

[Robin Murphy <robin.murphy@arm.com>]

On 2022-08-17 17:07, Vadym Kochan wrote:
> Hi Robin, Adrian,
> 
> On Wed, Aug 17, 2022 at 02:43:46PM +0100, Robin Murphy wrote:
>> On 2022-08-16 21:51, Vadym Kochan wrote:
>> [...]
>>>> The one thing to watch out for is that SWIOTLB doesn't necessarily interact
>>>> very well with DMA offsets. Given the intent of
>>>> of_dma_get_max_cpu_address(), I think it ought to work out OK now for
>>>> current kernels on DT systems if everything is described correctly, but
>>>> otherwise it's likely that you end up with ZONE_DMA either being empty or
>>>> containing all memory, so the SWIOTLB buffer ends up being allocated
>>>> anywhere such that it might not actually work as expected.
>>>>
>>>> Robin.
>>>
>>> Hi Robin,
>>>
>>> Thank you for the reply.
>>>
>>> My understanding is that swiotlb is allocated (in case of arm64)
>>> in the following cases:
>>>
>>>      #1 when it is forced from the kernel cmdline
>>>
>>>      #2 when max_pfn is greater than arm64_dma_phys_limit (and this is used
>>>         as the end from which to allocate the swiotlb pool in the
>>>         top-botom direction via memblock API).
>>>
>>>      #3 using restricted dma-pool
>>>
>>> Of course option #3 works fine because swiotlb is kind of forced to use
>>> particulary this range of memory.
>>>
>>> Both options #1 & #2 causes to use full memory mask even if to specify
>>> dma-ranges in the DT:
>>>
>>>       dma-ranges = <0x0 0x0 0x2 0x0 0x0 0x80000000>;
>>>
>>> or if to specify the opposite:
>>>
>>>       dma-ranges = <0x2 0x0 0x0 0x0 0x0 0x80000000>;
>>>
>>>       just to make it lower than U32 to pass
>>>
>>>           zone_dma_bits = min3(32U, dt_zone_dma_bits, acpi_zone_dma_bits)
>>>
>>>       condition, but then it will be re-set in max_zone_phys() by:
>>>
>>> 	if (phys_start > U32_MAX)
>>> 		zone_mask = PHYS_ADDR_MAX;
>>> 	else if (phys_start > zone_mask)
>>> 		zone_mask = U32_MAX;
>>
>> Ah, indeed I missed that, sorry. It seems that that change to stop assuming
>> an offset kind of crossed over with the introduction of
>> *_dma_get_max_cpu_address(), but now that that firmware property parsing
>> *is* implemented, in principle it should be equally possible to evaluate the
>> actual offsets as well, and decide whether an offset ZONE_DMA is appropriate
>> or not. Either way, this is definitely the area which needs work if we want
>> to to able to support topologies like this properly.
>>
>>> So, currently I dont see how to pin swiotlb (I see it as a main problem) to some specific range of physical
>>> memory (particulary to the first 2G of RAM).
>>
>> Indeed, if ZONE_DMA and/or ZONE_DMA32 can't be set appropriately, then
>> there's no way to guarantee correct allocation of any DMA buffers, short of
>> hacking it with explicitly placed reserved-memory carveouts.
>>
> 
> I have sent some time ago a solution which binds restricted-dma pool to
> the eMMC device, so Adrian, Robin do you think this can be acceptable as
> a temporary solution (at least conceptually) ?
> 
> I was also thinking would it be OK to introduce something like
> bounced-dma pool (similar to the restricted one) which will reserve
> memory for the bounced buffers only ? It should not be hard as looks
> like it will re-use existing interface between dma and swiotlb ? In that
> case it would allow to map first 2G of memory to eMMC controller.

TBH I'd prefer to fix it (or at least work around it) more generally.
Putting made-up things in devicetree to work around shortcomings in
kernel code tends to be a hole that's hard to dig yourself back out of.
As a bodge that would be just about justifiable in its own terms, does
the diff below help at all?

Thanks,
Robin.

----->8-----
diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index b9af30be813e..88f7b26f49db 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -451,7 +451,14 @@ void __init bootmem_init(void)
   */
  void __init mem_init(void)
  {
+	/*
+	 * Some platforms still manage to elude our attempt to calculate
+	 * ZONE_DMA appropriately, so encourage the SWIOTLB allocation to go
+	 * as low as it can anyway for the best chance of being usable.
+	 */
+	memblock_set_bottom_up(true);
  	swiotlb_init(max_pfn > PFN_DOWN(arm64_dma_phys_limit), SWIOTLB_VERBOSE);
+	memblock_set_bottom_up(false);
  
  	/* this will put all unused low memory onto the freelists */
  	memblock_free_all();