include: zephyr: drivers: Add flash_get_page_layout() syscall

[duda-patryk]

Each flash layout entry describes continuous part of flash with the same page size (page is a smallest erasable unit of flash). This way, the flash layout is represented in an efficient way e.g. if all flash pages have the same size, the whole flash will be described in one entry.

Introduced function provides direct access to the flash layout representation.

Best regards,
Patryk

[de-nordic]

Not sure this is good idea.
You just expose pointer to kernel/driver structure that may at some point happen in protected part of memory and cause protection fault when iterated over in user space.

[duda-patryk]

Actually there are couple of examples of this:

• flash_get_parameters() also returns pointer to internal driver structure.
• flash_page_foreach() is not marked as __syscall. It seems like userspace is calling api->page_layout() directly (and accessing driver structures).

But I understand your point. What do you think about adding flash_layout_foreach() marked as __syscall?

[de-nordic]

Actually there are couple of examples of this:

* `flash_get_parameters()` also returns pointer to internal driver structure.

* `flash_page_foreach()` is not marked as `__syscall`. It seems like userspace is calling `api->page_layout()` directly (and accessing driver structures).

Yes we have problems there. The flash_get_parameters should probably get modified to do copyout to provided buffer; worth thinking it over.

But I understand your point. What do you think about adding flash_layout_foreach() marked as __syscall?

I was considering that, but got two problems with this:

1. this will elevate execution of user code to system space;
2. if you make the callback CPU heavy you will end up spending a lot of time in syscall iterating over flash.

The foreach case is quite rare and there are three main uses now 1) get flash size, 2) check flash layout to figure out whether it is uniform, because FS needs that, 3) list all pages for some reason (shell?)

These shouldn't happen too often and can probably take performance hit in querying each page info individually.

Some time ago I proposed this #36484 which gets rid of the layout array and instead directly asks drivers for information on flash size or page info. Unfortunately it is in review limbo.
Above makes foreach less efficient, due to multiple syscalls to get info for each page independently, but allows to, for example, to query external flash devices for size without hardcoding it into the driver.