Re: [PATCH 0/2] efi_loader: provide media ID

[AKASHI Takahiro <takahiro.akashi@linaro.org>]

On Mon, Sep 26, 2022 at 08:06:52AM +0200, Heinrich Schuchardt wrote:
> 
> 
> On 9/16/22 02:58, AKASHI Takahiro wrote:
> > On Thu, Sep 15, 2022 at 10:02:40PM +0200, Heinrich Schuchardt wrote:
> > > The medium a device like 'mmc 0' or 'usb 0' points to may change over
> > > time. Hence device type and number are not sufficient to identify the
> > > inserted medium. The same is true for the device path generated for
> > > such a device.
> > 
> > Well, it depends on how a device path is generated in U-Boot's UEFI
> > implementation. I believe that a device path represents an "unique path"
> > to a given device however this device is enumerated.
> > In this sense, the current dp_fill()/efi_dp_from_part() is not a right
> > implementation as it relies on device numbers.
> > Furthermore, a generated device path here is different from one generated
> > by EDK2 (even if both software are run on the same board).
> > 
> > This is an issue that I used to tackle in
> > https://lists.denx.de/pipermail/u-boot/2021-November/468216.html
> > although I have since had no progress.
> > 
> > > This is why the EFI_BLOCK_IO_PROTOCOL provides a field
> > > MediaId.
> > > 
> > > Whenever a removable medium is changed or a new block device with a
> > > previously used device path is created we should provide a different
> > > MediaID.
> > > 
> > > This series adds a field media_id to the block device descriptor and fills
> > > it after probing. The value of the field is then copied to the
> > > EFI_BLOCK_IO_PROTOCOL.
> > 
> > I'm afraid that your patch doesn't always work as you expect.
> > When "scsi rescan" or "usb stop; usb start", for instance, is invoked,
> > all the existing devices and associated blk_desc structures are once freed
> > and even if nothing is changed, i.e. a device is neither removed nor added,
> > the exact same structures will be re-created.
> > With your patch applied, however, a new (and different) "media_id" will be
> > assigned to an existing device. UEFI User may be notified of "media change".
> > (To be honest, this is quite unlikely because the current UEFI implementation
> > doesn't use BLOCK_IO_PROTOCOL internally, say, for file system access.)
> 
> This behavior matches what EDK II does if you remove a device and create a
> new device.

I don't think that EDK2 has "scsi rescan" or others, which users can invoke
at any time. Moreover, I believe that EDK2 code (drivers) checks whether a device
is really changed or not before updating a MediaId.

> If a device is removed and recreated anything could have happened in between
> like complete repartitioning. We cannot assume that any cached state is
> valid anymore even if GUIDs are the same.

I'm not sure if you fully understand my point.
My assumption is the case where a device is NOT removed around "scsi rescan"
(or usb stop/start) and stays online. In this case,
1. access to, say, "scsi 0:1", via UEFI BLOCK_IO succeeds
2. "scsi rescan"
3. access to the same device, "scsi 0:1", via UEFI BLOCK_IO 
currently (3) succeeds, but with your patch, it may potentially fail because
of media_id altered.

I admit that it will not happen under the current UEFI implementation because
non of UEFI applications will survive across command lines and none of information,
including media_id or handle, can be carried over from (1) to (3).
But unconditionally incrementing an internally-held media_id, as in your patch,
is a wrong behavior.

-Takahiro Akashi

> 
> So it is correct to change the media ID in this case.

> Commands like scsi rescan are needed because we don't monitor media changes
> in the DM drivers yet. Simon's suggestion to use provide an event for media
> changes looks like the right approach to me.
> 
> Best regards
> 
> Heinrich
> 
> > 
> > -Takahiro Akashi
> > 
> > > With future patches we can refine this in sub-systems like USB, MMC, SCSI
> > > to indicate media changes
> > > 
> > > Heinrich Schuchardt (2):
> > >    dm: blk: assign media ID to block devices
> > >    efi_loader: fill media_id from block device descriptor
> > > 
> > >   drivers/block/blk-uclass.c | 16 +++++++++++++++-
> > >   include/blk.h              | 11 +++++++++++
> > >   lib/efi_loader/efi_disk.c  |  6 +-----
> > >   3 files changed, 27 insertions(+), 6 deletions(-)
> > > 
> > > -- 
> > > 2.37.2
> > >