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

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

Hi chiming in a little late but

On Mon, 8 Nov 2021 at 06:46, AKASHI Takahiro <takahiro.akashi@linaro.org> wrote:
>
> On Fri, Nov 05, 2021 at 10:12:16AM -0600, Simon Glass wrote:
> > Hi Takahiro,
> >
> > On Thu, 4 Nov 2021 at 20:49, AKASHI Takahiro <takahiro.akashi@linaro.org> wrote:
> > >
> > > On Thu, Nov 04, 2021 at 08:02:05PM -0600, Simon Glass wrote:
> > > > Hi,
> > > >
> > > > On Tue, 2 Nov 2021 at 01:43, Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:
> > > > >
> > > > >
> > > > >
> > > > > On 11/1/21 03:14, Simon Glass wrote:
> > > > > > Hi Takahiro,
> > > > > >
> > > > > > On Sun, 31 Oct 2021 at 19:52, AKASHI Takahiro
> > > > > > <takahiro.akashi@linaro.org> wrote:
> > > > > >>
> > > > > >> On Sun, Oct 31, 2021 at 07:15:17PM -0600, Simon Glass wrote:
> > > > > >>> Hi Takahiro,
> > > > > >>>
> > > > > >>> On Sun, 31 Oct 2021 at 18:36, AKASHI Takahiro
> > > > > >>> <takahiro.akashi@linaro.org> wrote:
> > > > > >>>>
> > > > > >>>> On Sat, Oct 30, 2021 at 07:45:14AM +0200, Heinrich Schuchardt wrote:
> > > > > >>>>>
> > > > > >>>>>
> > > > > >>>>> Am 29. Oktober 2021 23:17:56 MESZ schrieb Simon Glass <sjg@chromium.org>:
> > > > > >>>>>> Hi,
> > > > > >>>>>>
> > > > > >>>>>> On Fri, 29 Oct 2021 at 13:26, Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:
> > > > > >>>>>>>
> > > > > >>>>>>>
> > > > > >>>>>>>
> > > > > >>>>>>> Am 29. Oktober 2021 08:15:56 MESZ schrieb AKASHI Takahiro <takahiro.akashi@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.
> > > > > >>>>>
> > > > > >>>>> Are further MMC hw partitions, multiple SCSI LUNs and multiple NVME  namespaces already treated as separate BLK devices?
> > > > > >>>>
> > > > > >>>> Yes.
> > > > > >>>> What I meant to say is that, if we don't need a partition table 'udevice'
> > > > > >>>> for hw partitions, we don't need such a device for sw partitions neither.
> > > > > >>>>
> > > > > >>>> Meanwhile, what about UCLASS_FS? Why do we need this?
> > > > > >>>
> > > > > >>> We don't need it for our current discussion, but if we want to 'open'
> > > > > >>> the filesystem and keep the metadata around, rather than reading it
> > > > > >>> again every time we access a file, we might find it useful. Open files
> > > > > >>> could be children of the FS uclass, perhaps, if we go a step further
> > > > > >>> and create devices for them.
> > > > > >>
> > > > > >> Do you want to invent linux-like mount-point concepts or procfs?
> > > > > >> I remember that you didn't want to have child nodes under BLK devices.
> > > > > >> I'm getting confused about our goal.
> > > > > >
> > > > > > I think we are all a bit unsure.
> > > > > >
> > > > > > I think BLK devices can have children, sorry if I said the wrong thing
> > > > > > somewhere along the way. For example, a partition would be under a BLK
> > > > > > device, or a FS.
> > > > > >
> > > > > >> What should DM represent in U-Boot world?
> > > > > >
> > > > > > That is what we are trying to figure out.
> > > > > >
> > > > > > I think the minimum is to have a a way to represent partitions (s/w
> > > > > > and hw/). As I understand it, that's what we've been discussing.
> > > > >
> > > > > The discovery of hardware partitions is specific to the block device
> > > > > controller SCSI/MMC/ATA/NVMe. We currently do not provide any
> > > > > manipulation commands to create hardware partitions (e.g. NVMe
> > > > > namespaces, SCSI LUNs). This is why extracting a uclass for hardware
> > > > > partitions does not seem necessary.
> > > >
> > > > I can see the reasoning here. It might not stand the test of time but
> > > > how about we go with it for now? For MMC hardware partition we would
> > > > just end up with multiple BLK devices, like we do with SCSI LUNs at
> > > > present, which seems like it should work (with some code tweaks).
> > > >
> > > > >
> > > > > Software partitioning (MBR, GPT, ...) is independent of the harboring
> > > > > block device.
> > > > >
> > > > > We already have a set of drivers for software partition tables in disk/.
> > > > > Currently the available methods of the drivers are defined in
> > > > > U_BOOT_PART_TYPE referring to struct part_driver.
> > > > >
> > > > > Currently struct part_driver knows only the following methods:
> > > > >
> > > > > - get_info()
> > > > > - print()
> > > > > - test()
> > > > >
> > > > > These drivers should be ome a uclass.
> > > > >
> > > > > gpt.c and mbr.c allow to create and delete partitions. I think we should add
> > > > >
> > > > > - create_partition()
> > > > > - delete_partition()
> > > > >
> > > > > to the uclass methods.
> > > >
> > > > That sounds good to me, although since it is a partition uclass, we
> > > > can just use create() and delete().
> > >
> > > I don't know why we need a "partition table" device in the middle
> > > of DM hierarchy.
> > > I believe that it simply makes the view of DM tree complicated
> > > without any explicit benefit.
> >
> > Well we clearly have an API here. The partition uclass can:
> >
> > - hold the partition table in dev_get_uclass_priv()
> > - support a read() operation to read the partition
> > - support create() to rewrite the partition table
> > - support delete() to overwrite/erase the partition table
> >
> > Then it means that filesystems have the partition table as a parent
> > (unless they are whole-device filesystems), which makes sense
> >
> > So that's why I like the idea.
> >
> > Other than the extra complexity, is there anything else wrong with it?
>
> - First of all, a partition table doesn't look like a 'device' at all.
> - Second, a partition table is just static data for block devices.
>   IMO, even if we want to have this data, we can simply hold it
>   as some sort of attribute of the device, or maybe as a 'tag' which
>   I will introduce in the next version.
>
> -Takahiro Akashi
>

I don't know how this affect the code, but I agree with Akashi-san
here.  It's indeed useful to keep the partition table stored
somewhere,  but I think not showing them as part of the device tree is
more intuitive.

Thanks
/Ilias
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