[RFC 07/22] dm: blk: add UCLASS_PARTITION

[Simon Glass]

Hi,

On Fri, 29 Oct 2021 at 13:26, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>
>
>
> Am 29. Oktober 2021 08:15:56 MESZ schrieb AKASHI Takahiro <takahiro.akashi at linaro.org>:
> >On Fri, Oct 29, 2021 at 06:57:24AM +0200, Heinrich Schuchardt wrote:
> >>
> >>
> >> > I agree with Heinrich that we are better to leave BLK as it is, both
> >> > in name and meaning. I think maybe I am missing the gist of your
> >> > argument.
> >> >
> >> > If we use UCLASS_PART, for example, can we have that refer to both s/w
> >> > and h/w partitions, as Herinch seems to allude to below? What would
> >> > the picture look like the, and would it get us closer to agreement?
> >>
> >> In the driver model:
> >>
> >> A UCLASS is a class of drivers that share the same interface.
> >> A UDEVICE is a logical device that belongs to exactly one UCLASS and is
> >> accessed through this UCLASS's interface.
> >
> >Please be careful about "accessed through" which is a quite confusing
> >expression. I don't always agree with this view.
> >
> >> A hardware partition is an object that exposes only a single interface
> >> for block IO.
> >>
> >> A software partition is an object that may expose two interfaces: one
> >> for block IO, the other for file IO.
> >
> >Are you talking about UEFI world or U-Boot?
> >Definitely, a hw partitions can provide a file system
> >if you want.
> >It's a matter of usage.
> >
> >I remember that we had some discussion about whether block devices
> >on UEFI system should always have a (sw) partition table or not.
> >But it is a different topic.
> >
> >> The UEFI model does not have a problem with this because on a handle you
> >> can install as many different protocols as you wish. But U-Boot's driver
> >> model only allows a single interface per device. Up to now U-Boot has
> >> overcome this limitation by creating child devices for the extra interfaces.
> >
> >> We have the following logical levels:
> >>
> >> Controller      | Block device | Software Partition| File system
> >> ----------------+--------------+-------------------+------------
> >> NVMe Drive      | Namespace    | Partition 1..n    | FAT, EXT4
> >> ATA Controller  | ATA-Drive    |                   |
> >> SCSI Controller | LUN          |                   |
> >> MMC Controller  | HW-Partition |                   |
> >> MMC Controller  | SD-Card      |                   |
> >> USB-Node        | USB-Drive    |                   |
> >>
> >> In the device tree this could be modeled as:
> >>
> >> |-- Controller (UCLASS_CTRL)
> >> | |-- Block device / HW Partition (UCLASS_BLK)    (A)
> >> | | |-- Partition table (UCLASS_PARTITION_TABLE)  (B)
> >> | |   |-- Software Partition (UCLASS_BLK)
> >> | |     |-- File system (UCLASS_FS)
> >> | |
> >> | |-- Block device (UCLASS_BLK)
> >> |   |-- File system (UCLASS_FS)
> >
> >I don't know why we expect PARTITION_TABLE and FS to appear in DM tree.
> >What is the benefit?
> >(A) and (B) always have 1:1 relationship.
>
> No. You can have a bare device without a partition table.

I can have a DOS partition that covers the whole device, without a
partition table. This is supported in U-Boot and Linux.

>
> We have several partition table drivers: DOS, GPT, OSX, ... . In future we should also have one for the NOR Flash partitions. All of these drivers have a common interface. As the partition table type is mostly independent of the block device type we should use separate uclasses and udevices.
>
> >I also remember that you claimed that not all efi objects(handles and
> >protocols like SIMPE_FILE_SYSTEM_PROTOCOL) need to have corresponding
> >U-Boot counterparts in our 2019 discussion.
> >
> >If we *need* PARTITION_TALBLE, why don't we have HW_PARTITION_TABLE,
> >which should support other type of hw partitions as well?
>
> How hardware partitions, LUNs, ATA drives are enumerated is specific to the type of controller while the type of software partition table  is independent of the block device.
>
> >
> >|-- eMMC controller (UCLASS_MMC)
> >| |-- eMMC device1 / Physical media (UCLASS_HW_PARTITION_TABLE?)
> >|   |-- Block device / HW Partition:user data (UCLASS_BLK)
> >|   | |-- Partition table (UCLASS_PARTITION_TABLE)
> >|   |   |-- Software Partition (UCLASS_BLK)
> >|   |     |-- File system (UCLASS_FS)
> >|   |
> >|   |-- Block device / HW Partition:boot0 (UCLASS_BLK)
> >|   |-- Block device / HW Partition:boot1 (UCLASS_BLK)
> >          ...
> >| |-- eMMC device2 / Physical media (UCLASS_HW_PARTITION_TABLE?)
> >
> >|-- scsi controller (UCLASS_SCSI)
> >| |-- scsi disk / Physical media (UCLASS_HW_PARTITION_TABLE?)
> >|   |-- scsi LUN1 (UCLASS_HW_PARTITION_TABLE?)
> >|   | |-- Partition table (UCLASS_PARTITION_TABLE)
> >|   |   |-- Software Partition (UCLASS_BLK)
> >|   |-- scsi LUN2 (UCLASS_HW_PARTITION_TABLE?)
> >          ...
> >
> >(Here I ignored scsi buses/channels which make things more complicated.)
> >
> >This kind of complex hierarchy doesn't benefit anybody.
>
> All these levels exist already. We simply do not model them yet in the DM way.
>
> The device tree depth is the outcome of the udevice exposing always only a single interface defined by the uclass.
>
> The UEFI design allows installing multiple protocol interfaces on a single handle. This may result in simpler device trees in some cases.

Yes, the complexity has to go somewhere. With driver model I chose to
have a single interface per uclass, since it is simpler to understand,
no need to request a protocol for a device, etc.

Our current setup is similar to this

|-- Controller (UCLASS_MMC)
| |-- Block device (UCLASS_BLK)     - 'usual' HW partition
| |-- Block device (UCLASS_BLK)     - e.g. for a different HW partition*

* although I don't think the MMC code actually supports it - SCSI does though

We want to add devices for the partition table and the filesystem, so could do:

|-- Controller (UCLASS_MMC)
| |-- Block device (UCLASS_BLK)     - 'usual' HW partition (the whole device)
| | |-- Partition table (UCLASS_PART)  - DOS partition (or EFI)
| | | |-- Block device (UCLASS_BLK)  - partition 1
| | | | |-- Filesystem (UCLASS_FS) - DOS filesystem
| | | |-- Block device (UCLASS_BLK)  - partition 2
| | | | |-- Filesystem (UCLASS_FS) - ext5 filesystem
| |-- Block device (UCLASS_BLK)     - e.g. for a different HW
partition (the whole device)

This is similar to Heinrich's, but without the top-level
UCLASS_HW_PARTITION_TABLE which I am not sure is necessary.

It is compatible with what we have now and we could enable/disable the
extra devices with a Kconfig.

Regards,
Simon



> >
> >> UCLASS_PARTITION_TABLE would be for the drivers in disk/.
> >> UCLASS_FS would be for the drivers in fs/.
> >> UCLASS_BLK will be for any objects exposing raw block IO. A software
> >> partition does the same. It is created by the partition table driver as
> >> child of the partition table udevice.
> >>
> >> In this model an eMMC device will not be a UCLASS_BLK device because it
> >> does not expose block IO. It is the hardware partition that exposes this
> >> interface.
> >>
> >> The suggested model will allow a clean description of nested partition
> >> tables.
> >>
> >> In the UEFI world the software partition and its file system must be
> >> mapped to a single handle with device path node type HD(). For the
> >> parent block device we may create a child handle with partition number 0
> >> (HD(0)). For the partition table we will not create a handle.
> >>
> >> Best regards
> >>
> >> Heinrich