[PATCH 0/2] efi_loader: provide media ID

[Simon Glass]

Hi,

On Tue, 27 Sept 2022 at 00:53, Heinrich Schuchardt
<heinrich.schuchardt at canonical.com> wrote:
>
>
>
> On 9/27/22 03:51, AKASHI Takahiro wrote:
> > 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.
>
> The patch issues a new media ID if a new device is probed which only
> happens to have the same device number if another device of that number
> was removed before.
>
> Commands like 'usb scan' don't necessarily issue the same numbers to the
> same device as before the command if a new device has been attached in
> the meanwhile.
>
> Assuming that a new device contains the same medium as an old one
> because by chance it has the same device number is definitively unsafe.
>
> If a device is probed, we have to assume that it contains a new medium.

Sorry if I repeat myself, but this sort of thing should be handled in
the driver model code. Can we get some more progress on integrating
the EFI layer better?

Regards,
Simon