Re: [PATCH v3 4/5] blkmap: store type of blkmap device in corresponding structure

[Tobias Waldekranz <tobias@waldekranz.com>]

On mån, jan 20, 2025 at 19:30, Sughosh Ganu <sughosh.ganu@linaro.org> wrote:
> On Mon, 20 Jan 2025 at 17:55, Tobias Waldekranz <tobias@waldekranz.com> wrote:
>>
>> On mån, jan 20, 2025 at 16:20, Sughosh Ganu <sughosh.ganu@linaro.org> wrote:
>> > Add information about the type of blkmap device in the blkmap
>> > structure. Currently, the blkmap device is used for mapping to either
>> > a memory based block device, or another block device (linear
>> > mapping). Put information in the blkmap structure to identify if it is
>> > associated with a memory or linear mapped device. Which can then be
>> > used to take specific action based on the type of blkmap device.
>>
>> Is this restriction really necessary? Why should it not be possible to
>> setup a block map like this:
>>
>>  myblkmap:
>> .--------.      .-----.
>> | slice0 +------> RAM |
>> :--------:      '-----'     .-------.
>> | slice1 +------------------> eMMC0 |
>> :--------:      .-------.   '-------'
>> | slice2 +------> eMMC1 |
>> '........'      '-------'
>>
>> Linux's "device mapper", after which blkmaps are modeled, works in this
>> way.  I.e. a blkmap is just a collection of slices, and it is up to each
>> slice how its data is provided, meaning that the user is free to compose
>> their virtual block device in whatever way they need.
>
> The blkmap structure, the way it is designed, is pointing to the
> underlying block device. How can a single blkmap then be associated

The `struct udevice *blk` from `struct blkmap` is a reference to the
block device which represents the block map itself ("myblkmap" in the
picture above), not any lower device.

> with slices of different types? Would that not contravene with the
> idea of a block device associating with a blkmap?

For slices which are linear mappings (and are thus backed by some other
underlying block device), their reference to that lower device ("eMMC0"
and "eMMC1" above) is stored in the `struct udevice *blk` member of
`struct blkmap_linear`.

Slices which are backed by memory does not have any reference to a lower
device, but merely a pointer to the start of the mapping - `void *addr`
in `struct blkmap_mem`.

The overarching idea is that the block map does not have to know
anything about the implementation of how any individual slice chooses to
provide its data.  It only knows about their sizes and offsets.  Based
on that information, it simply routes incoming read/write requests to
the correct slice.

>>
>> Looking at the pmem patch that follows this one, I am not able to find
>> anything that would motivate restricting the functionality either.
>
> The subsequent patch is adding the persistent memory node to the
> device-tree. The pmem node that is to be added is the memory mapped
> blkmap device. The logic does check for the type of the blkmap device
> and then proceeds to add the pmem node only for the memory mapped
> blkmaps.

Sorry I am confused.  Why do you need a block map device to add the pmem
node to the device tree?