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

[Adrian Hunter]

On 8/11/22 21:05, Vadym Kochan wrote:
> Hi Adrian,
> 
> On Thu, 13 Oct 2022 09:40:00 +0300, Vadym Kochan <vadym.kochan@xxxxxxxxxxx> wrote:
>> Hi Robin,
>>
>> On Mon, 22 Aug 2022 11:06:43 +0100, Robin Murphy <robin.murphy@xxxxxxx> wrote:
>>> On 2022-08-21 07:17, Christoph Hellwig wrote:
>>>> On Thu, Aug 18, 2022 at 03:07:40PM +0300, Vadym Kochan wrote:
>>>>> It works with the following changes:
>>>>>
>>>>>      #1 dma-ranges = <0x0 0x0 0x2 0x0 0x0 0x80000000>;
>>>>>
>>>>>      #3 swiotlb="force"
>>>>>
>>>>> Is it OK to force the memory allocation from the start for the swiotlb ?
>>>>
>>>> It should be ok, but isn't really optimal.
>>>>
>>>> I wonder if we should just allow DT to specify the swiotlb buffer
>>>> location.  Basically have yet another RESERVEDMEM_OF_DECLARE variant
>>>> for it, which shouldn't be all that much work except for figuring
>>>> out the interaction with the various kernel command line options.
>>>
>>> We already have all the information we need in the DT (and ACPI), the 
>>> arm64 init code just needs to do a better job of interpreting it 
>>> properly. I'll see what I can come up with once I've finished what I'm 
>>> currently tied up in.
>>>
>>> Thanks,
>>> Robin.
>>
>> Sorry to disturb you, I just 'd like to know if you have
>> some ideas to share or patches to test ?
>>
>> Thank you!
>>
> 
> Since AC5X eMMC controller can fail to work on boards with >2GB memory,
> and considering that the best fix may not be easy (as it requires arm64 infra changes),
> so would it be OK to use PIO mode as temporary solution ?
> 
> I understand that arm64 changes might not be trivial and it might take significant
> amount of time to implement considering this unusual case, I just think that better
> to make eMMC working even if it will be slow.

You can disable DMA if you wish:
	SDHCI_QUIRK_BROKEN_DMA | SDHCI_QUIRK_BROKEN_ADMA
however did you consider SDMA:
	SDHCI_QUIRK_BROKEN_ADMA
which uses a bounce buffer allocated by SDHCI?

In any case, you need to add comments to the code
and commit message explaining the swiotlb issue.