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

[Heinrich Schuchardt <heinrich.schuchardt@canonical.com>]

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.

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.

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
>>