[RFC PATCH 3/6] block: add new genhd flag GENHD_FL_NO_NVMEM

[Hannes Reinecke]

On 7/20/23 16:28, Daniel Golle wrote:
> On Thu, Jul 20, 2023 at 04:03:22PM +0200, Hannes Reinecke wrote:
>> On 7/20/23 15:47, Daniel Golle wrote:
>>> On Thu, Jul 20, 2023 at 10:24:18AM +0200, Hannes Reinecke wrote:
>>>> On 7/20/23 00:03, Daniel Golle wrote:
>>>>> Add new flag to destinguish block devices which should not act as an
>>>>> NVMEM provider, such as for example an emulated block device on top of
>>>>> an MTD partition which already acts as an NVMEM provider itself.
>>>>>
>>>>> Signed-off-by: Daniel Golle <daniel at makrotopia.org>
>>>>> ---
>>>>>     include/linux/blkdev.h | 3 +++
>>>>>     1 file changed, 3 insertions(+)
>>>>>
>>>>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>>>>> index 2f5371b8482c0..e853d1815be15 100644
>>>>> --- a/include/linux/blkdev.h
>>>>> +++ b/include/linux/blkdev.h
>>>>> @@ -80,11 +80,14 @@ struct partition_meta_info {
>>>>>      * ``GENHD_FL_NO_PART``: partition support is disabled.  The kernel will not
>>>>>      * scan for partitions from add_disk, and users can't add partitions manually.
>>>>>      *
>>>>> + * ``GENHD_FL_NO_NVMEM``: NVMEM emulation is disabled.  The kernel will not
>>>>> + * emulate an NVMEM device on top of this disk.
>>>>>      */
>>>>>     enum {
>>>>>     	GENHD_FL_REMOVABLE			= 1 << 0,
>>>>>     	GENHD_FL_HIDDEN				= 1 << 1,
>>>>>     	GENHD_FL_NO_PART			= 1 << 2,
>>>>> +	GENHD_FL_NO_NVMEM			= 1 << 3,
>>>>>     };
>>>>>     enum {
>>>> Please reverse this flag. Most of the devices will not have an NVMEM
>>>> partition, and we shouldn't require each and every driver to tag their
>>>> devices.
>>>> So please use GENHD_FL_NVMEM and only set this flag on devices which really
>>>> have an NVMEM partition.
>>>
>>> The idea here was to exclude all those devices which already implement
>>> an NVMEM provider on a lower layer themselves, such as MTD.
>>> In this cases it would be ambigous if the OF node represents the
>>> NVMEM device registered by the MTD framework or if blk-nvmem should be
>>> used.
>>>
>> Hmm; not sure if I follow.
>> In the end, it doesn't really matter whether you check for
>> GENHD_FL_NO_NVMEM or !GENHD_FL_NVMEM.
>> With the difference being that in the former case you have to
>> tag 99% of all existing block devices, and in the latter you
>> have to tag 1%.
> 
> That's not exactly true. In the current case I only have to flag MTD
> (and UBI in future, I'm working on a UBI NVMEM provider as well) with
> GENHD_FL_NO_NVMEM, so a 'compatible = "nvmem-cells"' in the
> corresponding device tree node should not result in blk-nvmem creating
> an NVMEM device based on the (mtd/ubi)block device, simply because the
> MTD framework (and UBI in future) will already have created their own
> NVMEM device attached to the very same device tree node.
> 
> In all other cases of block devices, the compatible string can be used
> to unambigously decide whether an NVMEM device should be created or
> not. blk-nvmem is opt-in, so unless the device is flagged by
> 'compatible = "nvmem-cells"' it will not do anything.
> 
> For all devices which anyway do not have any device tree representation
> it won't do anything (think: loop, nbd, ...), we would not need to opt
> them out using GENHD_FL_NO_NVMEM. Also all other drivers which do not
> already bring their own NVMEM implementation won't need GENHD_FL_NO_NVMEM,
> the absence of 'compatible = "nvmem-cells"' is enough to indicate that
> they should not be considered as NVMEM providers.
> 
> The way you are suggesting will require that, in addition to selecting
> the targetted block device in device tree, the block driver will also
> have to set GENHD_FL_NVMEM. Hence we will need changes in MMC, NVMe
> and potentially also SATA disk drivers setting GENHD_FL_NVMEM when
> registering the disk.
> 
That is absolutely correct, and was my intention all along.
Drivers which can (and do) supply an NVMEM partition should be required 
to set this flag, yes.

Cheers,

Hannes