[resent RFC 00/22] efi_loader: more tightly integrate UEFI disks to device model

Heinrich Schuchardt xypron.glpk at gmx.de
Mon Oct 4 16:47:53 CEST 2021

On 10/4/21 05:44, AKASHI Takahiro wrote:
> # Resending the RFC as some of patches were deplicately submitted.
> # See also
>    https://git.linaro.org/people/takahiro.akashi/u-boot.git efi/dm_disk
> The purpose of this RPC is to reignite the discussion about how UEFI
> subystem would best be integrated into U-Boot device model.
> In the past, I poposed a couple of patch series, the latest one[1],
> while Heinrich revealed his idea[2], and the approach taken here is
> something between them, with a focus on block device handlings.
> # The code is a PoC and not well tested yet.
> Disks in UEFI world:
> ====================
> In general in UEFI world, accessing to any device is performed through
> a 'protocol' interface which are installed to (or associated with) the device's
> UEFI handle (or an opaque pointer to UEFI object data). Protocols are
> implemented by either the UEFI system itself or UEFI drivers.
> For block IO's, it is a device which has EFI_BLOCK_IO_PROTOCOL (efi_disk
> hereafter). Currently, every efi_disk may have one of two origins:
> a.U-Boot's block devices or related partitions
>    (lib/efi_loader/efi_disk.c)
> b.UEFI objects which are implemented as a block device by UEFI drivers.
>    (lib/efi_driver/efi_block_device.c)
> All the efi_diskss as (a) will be enumelated and created only once at UEFI
> subsystem initialization (efi_disk_register()), which is triggered by
> first executing one of UEFI-related U-Boot commands, like "bootefi",
> "setenv -e" or "efidebug".
> EFI_BLOCK_IO_PROTOCOL is implemented by UEFI system using blk_desc(->ops)
> in the corresponding udevice(UCLASS_BLK).
> On the other hand, efi_disk as (b) will be created each time UEFI boot
> services' connect_controller() is executed in UEFI app which, as a (device)
> controller, gives the method to access the device's data,
>>>> more details >>>
> Internally, connect_controller() search for UEFI driver that can support
> this controller/protocol, 'efi_block' driver(UCLASS_EFI) in this case,
> then calls the driver's 'bind' interface, which eventually installs
> the controller's EFI_BLOCK_IO_PROTOCOL to efi_disk object.
> 'efi_block' driver also create a corresponding udevice(UCLASS_BLK) for
>    * creating additional partitions efi_disk's, and
>    * supporting a file system (EFI_SIMPLE_FILE_SYSTEM_PROTOCOL) on it.
> <<< <<<
> Issues:
> =======
> 1. While an efi_disk represents a device equally for either a whole disk
>     or a partition in UEFI world, the device model treats only a whole
>     disk as a real block device or udevice(UCLASS_BLK).
> 2. efi_disk holds and makes use of "blk_desc" data even though blk_desc
>     in plat_data is supposed to be private and not to be accessed outside
>     the device model.
>     # This issue, though, exists for all the implmenetation of U-Boot
>     # file systems as well.
> For efi_disk(a),
> 3. A block device can be enumelated dynamically by 'scanning' a device bus
>     in U-Boot, but UEFI subsystem is not able to update efi_disks accordingly.
>     For examples,
>      => scsi rescan; efidebug devices
>      => usb start; efidebug devices ... (A)
>     (A) doesn't show any usb devices detected.
>      => scsi rescan; efidebug boot add -b 0 TEST scsi 0:1 ...
>      => scsi rescan ... (B)
>      => bootefi bootmgr ... (C)
>     (C) may de-reference a bogus blk_desc pointer which has been freed by (B).
>     (Please note that "scsi rescan" removes all udevices/blk_desc and then
>      re-create them even if nothing is changed on a bus.)
> For efi_disk(b),
> 4. A controller (handle), combined with efi_block driver, has no
>     corresponding udevice as a parent of efi_disks in DM tree, unlike, say,
>     a scsi controller, even though it provides methods for block io perations.
> 5. There is no way supported to remove efi_disk's even after
>     disconnect_controller() is called.
> My approach in this RFC:
> ========================
> Due to functional differences in semantics, it would be difficult
> to identify "udevice" structure as a handle in UEFI world. Instead, we will
> have to somehow maintain a relationship between a udevice and a handle.
> 1-1. add a dedicated uclass, UCLASS_PARTITION, for partitions
>     Currently, the uclass for paritions is not a UCLASS_BLK.
>     It can be possible to define partitions as UCLASS_BLK
>     (with IF_TYPE_PARTION?), but
>     I'm afraid that it may introduce some chaos since udevice(UCLASS_BLK)
>     is tightly coupled with 'struct blk_desc' data which is still used
>     as a "structure to a whole disk" in a lot of interfaces.
>     (I hope that you understand what it means.)
>     In DM tree, a UCLASS_PARTITON instance has a UCLASS_BLK parent:
>     For instance,
> 			 (IF_TYPE_SCSI)        |
>                            +- struct blk_desc   +- struct disk_part
> 			  +- scsi_blk_ops      +- blk_part_ops
> 1-2. create partition udevices in the context of device_probe()
>     part_init() is already called in blk_post_probe(). See the commit
>     d0851c893706 ("blk: Call part_init() in the post_probe() method").
>     Why not enumelate partitions as well in there.
> 2. add new block access interfaces, which takes "udevice" as a target device,
>     in U-Boot and use those functions to implement efi_disk operations
> 3-1. maintain a bi-directional link by adding
>     - a UEFI handle pointer in "struct udevice"
>     - a udevice pointer in UEFI handle (in fact, in "struct efi_disk_obj")

An EFI application can create handles with any combination of protocols,
e.g. a handle with both the block IO protocol and the simple network
protocol. This means that a udevice cannot be assigned to a handle
created by an EFI application.

When the EFI application calls ConnectController() for the handle,
U-Boot can create child controllers. If U-Boot creates a udevice for
such a child controller, it has to store the udevice pointer.
lib/efi_driver/efi_block_device.c uses a private data section but you it
could be preferable to use a field in struct efi_obj.

> 3-2. use device model's post_probe/pre_remove hook to synchronize the lifetime
>     of efi_disk objects in UEFI world with the device model.
> 4. I have no answer to issue(4) and (5) yet.

4) A udevice shall only exist for the child controller handle created by
U-Boot and not for the controller handle created by an EFI application.

5) The stop() method of the driver binding protocol has to take care of
destroying the child controllers and the associated udevices.

Best regards


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