From: Damien Le Moal <Damien.LeMoal@wdc.com>
To: Sean Anderson <seanga2@gmail.com>,
"linux-riscv@lists.infradead.org"
<linux-riscv@lists.infradead.org>,
Palmer Dabbelt <palmer@dabbelt.com>
Cc: Anup Patel <Anup.Patel@wdc.com>,
Paul Walmsley <paul.walmsley@sifive.com>
Subject: Re: [PATCH 02/10] riscv: Force flat memory model with no-mmu
Date: Sat, 15 Feb 2020 02:40:59 +0000 [thread overview]
Message-ID: <BYAPR04MB5816D6AB39649A563BC96F2FE7140@BYAPR04MB5816.namprd04.prod.outlook.com> (raw)
In-Reply-To: cb38129d-ceb8-4eb0-6bbb-a9c825478410@gmail.com
On 2020/02/15 11:26, Sean Anderson wrote:
> On 2/14/20 9:15 PM, Damien Le Moal wrote:
>> On 2020/02/15 5:18, Sean Anderson wrote:
>>> Hi,
>>>
>>> On 2/12/20 5:34 AM, Damien Le Moal wrote:
>>>> Compilation errors trigger if ARCH_SPARSEMEM_ENABLE is enabled for
>>>> a nommu kernel. Since the sparsemem model does not make sense anyway
>>>> for the nommu case, do not allow selecting this option to always use
>>>> the flatmem model.
>>>>
>>>> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
>>>> ---
>>>> arch/riscv/Kconfig | 1 +
>>>> 1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>>>> index 73f029eae0cc..1a3b5a5276be 100644
>>>> --- a/arch/riscv/Kconfig
>>>> +++ b/arch/riscv/Kconfig
>>>> @@ -121,6 +121,7 @@ config ARCH_FLATMEM_ENABLE
>>>>
>>>> config ARCH_SPARSEMEM_ENABLE
>>>> def_bool y
>>>> + depends on MMU
>>>> select SPARSEMEM_VMEMMAP_ENABLE
>>>>
>>>> config ARCH_SELECT_MEMORY_MODEL
>>>>
>>>
>>> Just for some background, why did you choose NOMMU? Afaik the K210 has
>>> an MMU following the RISC-V privileged specification 1.9
>>
>> Our early experiments with the k210 with opensbi revealed that the mmu is
>> definitely not a normal one or that it is not functional (e.g. S-mode fault
>> delegation bit setup leads to a hang). So at the time, we started assuming
>> that this is a nommu platform.
>>
>> Since then, others also mentioned that there is in fact an MMU but not
>> following the latest specs (I think Olof mentioned that). But I have not
>> look into this (yet) to try to make it work. Not sure how much effort would
>> be needed on the kernel to support this older specs mmu.
>>
>> In any case, considering the tiny 6+2MB of memory available, direct M-mode
>> Linux boot avoids the bootloader chain and openSBI use, which saves a lot
>> of memory. We could reduce this chain to opensbi with direct payload only,
>> but even then, page alignment will lead to memory loss. And at run-time,
>> nommu saves a lot too with the absence of page tables. Nommu makes sense
>> for this platform.
>
> Well, the VM mode bits are in mstatus for this priv spec, so OpenSBI
> won't work since there is no way to set them.
Interesting. At FOSDEM, we discussed with Palmer the work that would be
needed to disentangle NOMMU and M-MODE boot, which for now are rather
synonymous, but shouldn't. I guess this platform would still require M-MODE
boot, but not necessarily NOMMU.
>> This is the first step to get this platform running Linux. Due to the low
>> memory, it probably isn't a practical use case to use Linux in the first
>> place, but it definitely is a great inexpensive platform for getting
>> started with RISCV. NOMMU allows running Linux without much effort. Going
>> forward, we can also try to get that SoC MMU running.
>
> Yeah, that's pretty reasonable. However, I don't think much has changed
> other than the locations of some of the registers has been changed
> around. The existing code to set up page table entries should not need
> major modifications.
OK. Sounds easy enough. But I think cleanup work to dissociate M-mode boot
and NOMMU will be needed first. After that, trying to enable the MMU should
be easier.
>
> Alternatively, the base+bound scheme could probably work pretty well
> with low memory, though we would not be able to re-use any existing
> code.
>
> --Sean
>
--
Damien Le Moal
Western Digital Research
next prev parent reply other threads:[~2020-02-15 2:41 UTC|newest]
Thread overview: 89+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-12 10:34 [PATCH 00/10] Kendryte k210 SoC boards support Damien Le Moal
2020-02-12 10:34 ` [PATCH 01/10] riscv: Fix gitignore Damien Le Moal
2020-02-20 0:15 ` Palmer Dabbelt
2020-02-12 10:34 ` [PATCH 02/10] riscv: Force flat memory model with no-mmu Damien Le Moal
2020-02-14 20:18 ` Sean Anderson
2020-02-15 2:15 ` Damien Le Moal
2020-02-15 2:26 ` Sean Anderson
2020-02-15 2:40 ` Damien Le Moal [this message]
2020-03-02 3:48 ` Anup Patel
2020-03-04 18:38 ` Palmer Dabbelt
2020-02-12 10:34 ` [PATCH 03/10] riscv: Unaligned load/store handling for M_MODE Damien Le Moal
2020-03-02 3:57 ` Anup Patel
2020-03-04 19:28 ` Palmer Dabbelt
2020-02-12 10:34 ` [PATCH 04/10] riscv: Add BUILTIN_DTB support Damien Le Moal
2020-03-02 3:58 ` Anup Patel
2020-03-04 19:28 ` Palmer Dabbelt
2020-03-05 4:58 ` Anup Patel
2020-03-05 5:14 ` Damien Le Moal
2020-03-05 5:37 ` Anup Patel
2020-03-05 6:13 ` Damien Le Moal
2020-03-08 6:10 ` Anup Patel
2020-03-05 8:18 ` Atish Patra
2020-03-07 0:02 ` Sean Anderson
2020-03-07 1:51 ` Atish Patra
2020-03-07 2:08 ` Sean Anderson
2020-03-06 23:56 ` Sean Anderson
2020-02-12 10:34 ` [PATCH 05/10] riscv: Add SOC early init support Damien Le Moal
2020-03-04 19:28 ` Palmer Dabbelt
2020-02-12 10:34 ` [PATCH 06/10] riscv: Add Kendryte K210 SoC support Damien Le Moal
2020-02-14 20:31 ` Sean Anderson
2020-03-04 19:38 ` Palmer Dabbelt
2020-02-12 10:34 ` [PATCH 07/10] riscv: Select required drivers for Kendryte SOC Damien Le Moal
2020-03-02 3:59 ` Anup Patel
2020-03-04 19:44 ` Palmer Dabbelt
2020-02-12 10:34 ` [PATCH 08/10] riscv: Add Kendryte K210 device tree Damien Le Moal
2020-02-14 20:51 ` Sean Anderson
2020-02-15 2:34 ` Damien Le Moal
2020-02-15 2:48 ` Sean Anderson
2020-02-15 3:00 ` Damien Le Moal
2020-02-18 14:12 ` Carlos Eduardo de Paula
2020-02-18 14:18 ` Sean Anderson
2020-02-18 14:30 ` Carlos Eduardo de Paula
2020-02-18 17:48 ` Sean Anderson
2020-02-18 19:26 ` Carlos Eduardo de Paula
2020-02-19 9:06 ` Wladimir J. van der Laan
2020-02-19 22:28 ` Sean Anderson
2020-02-20 10:48 ` Wladimir J. van der Laan
2020-02-22 19:07 ` Wladimir J. van der Laan
2020-04-01 17:55 ` Drew Fustini
2020-04-02 2:24 ` Damien Le Moal
2020-02-19 8:50 ` Wladimir J. van der Laan
2020-02-27 19:43 ` Sean Anderson
2020-03-02 4:06 ` Anup Patel
2020-03-02 4:15 ` Damien Le Moal
2020-03-02 4:22 ` Anup Patel
2020-03-02 4:51 ` Damien Le Moal
2020-03-02 5:05 ` Anup Patel
2020-03-02 5:08 ` Damien Le Moal
2020-03-07 0:18 ` Sean Anderson
2020-03-07 4:11 ` Anup Patel
2020-03-04 19:44 ` Palmer Dabbelt
2020-02-12 10:34 ` [PATCH 09/10] riscv: Kendryte K210 default config Damien Le Moal
2020-03-02 4:07 ` Anup Patel
2020-03-04 19:44 ` Palmer Dabbelt
2020-02-12 10:34 ` [PATCH 10/10] riscv: create a loader.bin for the kendryte kflash.py tool Damien Le Moal
2020-03-02 4:08 ` Anup Patel
2020-03-04 19:44 ` Palmer Dabbelt
2020-02-14 15:05 ` [PATCH 00/10] Kendryte k210 SoC boards support Carlos Eduardo de Paula
2020-02-15 2:02 ` Damien Le Moal
2020-02-17 13:28 ` Carlos Eduardo de Paula
2020-02-26 21:31 ` Carlos Eduardo de Paula
2020-02-27 2:18 ` Damien Le Moal
2020-02-28 20:32 ` Sean Anderson
2020-03-02 3:01 ` Damien Le Moal
2020-03-02 3:53 ` Sean Anderson
2020-03-02 4:11 ` Damien Le Moal
2020-03-02 4:18 ` Sean Anderson
2020-03-02 4:54 ` Damien Le Moal
2020-03-02 4:56 ` Sean Anderson
2020-03-02 5:03 ` Damien Le Moal
2020-03-02 4:17 ` Anup Patel
2020-03-02 4:21 ` Sean Anderson
2020-03-02 4:48 ` Damien Le Moal
2020-03-02 4:51 ` Damien Le Moal
2020-03-02 5:02 ` Sean Anderson
2020-03-02 5:11 ` Damien Le Moal
2020-03-02 5:25 ` Sean Anderson
2020-03-02 5:43 ` Damien Le Moal
2020-03-04 19:44 ` Palmer Dabbelt
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=BYAPR04MB5816D6AB39649A563BC96F2FE7140@BYAPR04MB5816.namprd04.prod.outlook.com \
--to=damien.lemoal@wdc.com \
--cc=Anup.Patel@wdc.com \
--cc=linux-riscv@lists.infradead.org \
--cc=palmer@dabbelt.com \
--cc=paul.walmsley@sifive.com \
--cc=seanga2@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).