On 09/05/2024 14:59, Sam Day wrote:
'Ello Sumit,
On Wednesday, 8 May 2024 at 4:48 PM, Sumit Garg <sumit.g...@linaro.org> wrote:
On Wed, 8 May 2024 at 18:09, Caleb Connolly caleb.conno...@linaro.org wrote:
Hi Sam,
On 08/05/2024 13:40, Sam Day wrote:
Salutations Sumit,
On Wednesday, 8 May 2024 at 11:14 AM, Sumit Garg sumit.g...@linaro.org wrote:
Hi Sam,
On Wed, 8 May 2024 at 00:11, Sam Day m...@samcday.com wrote:
The newly introduced carve_out_reserved_memory causes issues when
U-Boot is chained from the lk2nd bootloader. lk2nd provides a
simple-framebuffer device and marks the framebuffer region as no-map in
the supplied /reserved-memory. Consequently, the simple_video driver
triggers a page fault when it tries to write to this region.
How does the corresponding Linux kernel driver handle this?
Firstly: I'm something of a middle-man here. I would consider Caleb the
authoritative source on the carveouts stuff (since they wrote it) and Nikita
Travkin the authority on the simplefb handoff (since he originally wrote it for
lk2nd to hand off to Linux simpledrm and then adapted it to work with U-Boot
simplefb).
I consulted with Nikita on your first question here. He linked me to this
snippet:
https://elixir.bootlin.com/linux/v6.9-rc7/source/drivers/gpu/drm/tiny/simpledrm.c#L877
Is the
framebuffer region required to be mapped as normal memory or device
type or something else?
So I guess based on the link above, it's just mapped as normal uncached memory.
I tried to do something like this a few days ago in U-Boot, but a) it doesn't
work and b) I have no idea what I'm doing:
https://github.com/samcday/u-boot/commit/c100cb3711ddf5b01601691f3e6a9ec890d9a496
Can you start adding some debug prints as to what might be happening?
Also, look at my proposal below to hook memory mapping properly for
drivers.
I got to the bottom of the issue. This commit seems to work:
https://github.com/samcday/u-boot/commit/25bf923c251b56d297d16898210fa36616294415
Calling mmu_set_region_dcache_behaviour alone is insufficient, as it
does not change the "flags" of the page-table entry in the way that
mmu_change_region_attr does. That's why it was triggering an abort
in the previous attempt: __asm_flush_dcache_range was trying to scrub
an entry that was still marked as PTE_TYPE_FAULT I guess :)
After talking with Caleb about this for a bit, they suggested the patch you see here as
what I guess could be considered a "stopgap" solution that hopefully makes it
into 2024.07.
Similarly would normal memory type work for
all other reserved memory regions marked as no-map?
I'll let Caleb weigh in here. My understanding is that the other regions should
be marked as PTE_TYPE_FAULT because otherwise drivers might inadvertently
speculatively access regions that are very much off-limits, such as TZ app
regions.
Right, carving out reserved regions and having the driver handle them is
theoretically the "correct" thing to do here. But I'm not sure that the
additional complexity offers much value from a U-Boot context.
If we can teach the armv8 PT/cache code to handle this better, and teach
all the drivers to map their regions on-demand, this would probably help
us a lot (especially as right now if you attempt to access dead space
between peripherals then you'll hang the bus...). But U-Boot is a ways
away from that.
I am not sure if U-Boot is really that far away although you can say
the infrastructure is not hooked up for Arm. Have a look into
map_physmem() and unmap_physmem() utilized by various drivers which
are actually hooked up on MIPS. So I think the right way to approach
this feature on Qcom platforms is to start hooking up those APIs on
mach-snapdragon to begin with via mmu_set_region_dcache_behaviour()
underneath. We can always go ahead and make it more generic if needed
on other platforms too.
So I'm happy to explore this further in a separate patch. Such a patch
doesn't seem like something that is suitable for inclusion in 2024.07
at this stage, right?
If not, I'm wondering if there's any chance we can still pull this patch
in as a stopgap measure? As it currently stands, the carveouts call is
the *only* thing preventing msm8916 devices from booting U-Boot when
chained from lk2nd.
This patches fixes simplefb on all Qualcomm devices, I'm going to pull
it in and we can pursue other solutions down the line.
Thanks both,
-Sam
-Sumit
--
// Caleb (they/them)
