[PATCH v1 2/5] drivers: nvme: Implement NVMe flush command (0x0)

[Neil Armstrong]

On 6/5/26 05:43, dmukhin at ford.com wrote:
> Hi Neil,
> 
> Thanks for your feedback!
> 
> Sorry, it took me a while to respond.
> 
> On Fri, May 29, 2026 at 09:54:32AM +0200, Neil Armstrong wrote:
> [..]
>>>
>>>>
>>>> The for NVMe, can it support write-through like SCSI FUA (Force Unit
>>>> Access) which bypasses the cache on writes, with slower writes
>>>> but safer and simpler than calling a flush before OS boot.
>>>
>>> Yes, I've seen there was FUA patch [1], which was then reverted [2].
>>>
>>> For our purposes, flush seems to be enough: the use case is
>>> updating NVMe-backed boot counter from a custom bootflow.
>>
>> OK the FUA was reverted because Bin's comment was not adressed:
>> https://lore.kernel.org/u-boot/CAEUhbmU02GnqEEfuqNrSj8R5XEZkmkTUaLp1+BMbYQNdzTUZ7A@mail.gmail.com/
>>
>> I would prefer having FUA added so all board could have this enabled
>> and not have to update their boot flow to call flush, this was basically
>> why the SCSI FUA was enabled.
> 
> I will take a look into FUA.
> 
> The problem I was solving is providing a guarantee that GPT headers
> will be updated correctly on the NVMe device.
> 
> I have a new bootcount device which implements a counter backed by
> GPT PTE flags (the top 16 bits) [1]
> 
> Patch [2] adds new APIs to update 16 bit flags in both primary and
> backup GPT headers (the proposed write_gpt_pte() call).
> 
> This is where I originally used new flush command: to guarantee that
> NVMe received all the updates before it is handed over to the OS.

I understand that, this is why FUA or disabling WC is exactly the same,
will beneficiate to all platforms and avoid calling flush everytime at the
cost of some small performance penalty since the NVMe driver isn't performance
optimized anyway.

Neil

> 
> [1] https://lore.kernel.org/u-boot/20260605033907.1909319-1-dmukhin@ford.com/
> [2] https://lore.kernel.org/u-boot/20260605033907.1909319-3-dmukhin@ford.com/
> 
>>
>> As Bin's comment, perhaps a way would be to disable the Write Cache if
>> the cache is present as init time (WCE), and it's even simpler, or
>> get the enable state of the WC and use FUA if enabled.
>>
>> Because the same question remains with the flush command, it's
>> only valid if the WC is present and is enabled.
>>
>> Thanks,
>> Neil
>>
>>>
>>> [1] https://lore.kernel.org/u-boot/20211019104049.v3.1.Ic581ec99f46b6dfa2e0b1922e670a333ac859e82@changeid/
>>> [2] https://lore.kernel.org/u-boot/CAGi-RUL96GK5nfHx73UQQd_4sFByt3gNwqhFpHps+h58S20JwQ@mail.gmail.com/
> 
> --
> Denis