Re: Troubles running Xen on Raspberry Pi 4 with 5.6.1 DomU

[Julien Grall <julien@xen.org>]

Hi,

On 13/05/2020 01:33, Stefano Stabellini wrote:
> I worked with Roman to do several more tests and here is an update on
> the situation. We don't know why my patch didn't work when Boris' patch
> [1] worked.  Both of them should have worked the same way.
> 
> Anyway, we continued with Boris patch to debug the new mmc error which
> looks like this:
> 
> [    3.084464] mmc0: unrecognised SCR structure version 15
> [    3.089176] mmc0: error -22 whilst initialising SD card
> 
> I asked to add a lot of trancing, see attached diff.

Please avoid attachment on mailing list and use pastebin for diff.

> We discovered a bug
> in xen_swiotlb_init: if io_tlb_start != 0 at the beginning of
> xen_swiotlb_init, start_dma_addr is not set correctly. This oneline
> patch fixes it:
> 
> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> index 0a40ac332a4c..b75c43356eba 100644
> --- a/drivers/xen/swiotlb-xen.c
> +++ b/drivers/xen/swiotlb-xen.c
> @@ -191,6 +191,7 @@ int __ref xen_swiotlb_init(int verbose, bool early)
>           * IO TLB memory already allocated. Just use it.
>           */
>          if (io_tlb_start != 0) {
> +               start_dma_addr = io_tlb_start;
>                  xen_io_tlb_start = phys_to_virt(io_tlb_start);
>                  goto end;
>          }
> 
> Unfortunately it doesn't solve the mmc0 error.
> 
> 
> As you might notice from the logs, none of the other interesting printks
> printed anything, which seems to mean that the memory allocated by
> xen_swiotlb_alloc_coherent and mapped by xen_swiotlb_map_page should be
> just fine.
> 
> I am starting to be out of ideas. Do you guys have any suggestions on
> what could be the issue or what could be done to discover it?

Sorry if my suggestions are going to be obvious, but I can't confirm 
whether this was already done:
     1) Does the kernel boot on baremetal (i.e without Xen)? This should 
help to confirm whether the bug is Xen is related.
     2) Swiotlb should not be necessary for basic dom0 boot on Arm. Did 
you try to disable it? This should help to confirm whether swiotlb is 
the problem or not.
     3) Did you track down how the MMC read the SCR structure?

Cheers,

-- 
Julien Grall