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
