Re: Keystone Issue

[Julien Grall <julien@xxxxxxx>]

Hi,

On 04/06/2020 10:08, Bertrand Marquis wrote:
> I would have thought that linux would have need some memory, even small in the 
> 32bit space in order to boot.
Yes it needs some, but then they are switching to use the high memory 
alias after the MMU has been switch on.
From my understanding, the only difference is the page-tables will 
point to the high memory alias address rather than the low memory one.
Linux will still be located at the same place but now accessed from the 
high memory alias rather than the low one.
Note that AFAICT the secondary CPUs will still be brought-up using the 
low memory alias.
> I could understand that some memory in the low address space needs to be 
> reserved by Linux as DMA area for peripherals not supporting 36-bit addresses, 
> but the whole low memory sounds like a big restriction.
Many platforms have devices only supporting 32-bit DMA, but none of them 
require such aliasing. So this doesn't look to be the issue here.
TBH, this code is only used by Keystone and switching address space is 
expensive (you have to turn off the MMU, updates page-tables, flush the 
cache...). I find hard to believe a developper would have come up with 
this complexity if it were possible to always use the low memory address 
range. It is even harder to believe Linux community would have accepted it.
> Would it be possible to have a bit more information on the “problem with 
> peripherals” here ?
I am curious as well, so I looked in more depth :). Going through the 
Linux history, one of the commit message [1] suggests they are switching 
to a coherent address space. The datasheet [2] (page 75) also confirm 
that the low region is not IO coherent.
So I think you would not be able to do DMA without flush the cache which 
can be pretty expensive. For a PoC, it might be possible to force Linux 
flushing the area before and after each DMA request. This should be 
possible by marking the devices as not coherent.
Although, I am not entirely sure if there is any fallout.

@Dave, do you think it is possible for you to have a try? I can provide 
the patch for Linux to disable DMA coherency if possible.
For a proper solution, I think we need to implement something similar to 
what I wrote earlier.
Cheers,

[1] 5eb3da7246a5b2dfac9f38a7be62b1a0295584c7
[2] https://www.ti.com/lit/ds/symlink/tci6638k2k.pdf?ts=1591183242813


--
Julien Grall