[PATCH] cmd: sf: Support unaligned flash updates with 'sf update'

[Frieder Schrempf]

On 30.09.21 23:08, Michael Walle wrote:
> Am 2021-09-30 19:17, schrieb Frieder Schrempf:
>> On 30.09.21 18:35, Michael Walle wrote:
>>> Am 2021-09-30 18:19, schrieb Frieder Schrempf:
>>>> In order to support unaligned updates, we simply read the first full
>>>> block and check only the requested part of the block for changes. If
>>>> necessary, the block is erased, the first (unchanged) part of the block
>>>> is written back together with the second part of the block (updated
>>>> data).
>>>
>>> I'm not sure what I should think of this. You might loose that unchanged
>>> part if there is an power loss in the middle, even worse, you likely
>>> don't
>>> have this part anymore because it isn't part of the data you want to
>>> write
>>> to the flash.
>>>
>>> Maybe add an parameter for allow (unsafe) unaligned updates?
>>>
>>> Now you might argue, that with "sf erase, sf write" you can do the same
>>> harm, to which i reply: but then it is intentional ;) Because "sf erase"
>>> just works on sector boundaries (which isn't really checked in the
>>> command,
>>> i just realized, but at least my driver returns EINVAL) and then the
>>> user has to include the "unchanged part" for the arguments on the
>>> commandline.
>>
>> True, but "sf update" already is "unsafe" even without supporting
>> unaligned start offsets. The code already handles partial sector writes
>> for the last sector in the same way (read, erase, write), which is also
>> prone to data loss in case of power loss.
> 
> Ah, I missed that. Yes, in this case, we don't loose anything. Agreed.
> 
>> So this patch in fact just adds support for partial sector updates for
>> the first sector. It is slightly more probable to loose data, but it
>> doesn't introduce new "unsafe" behavior.
>>
>> Maybe we could cover this by adding a warning to the documentation, or
>> even printing a warning?
> 
> Heh, although I was using "sf update" all the time, I wasn't aware of
> the read - "partly modify" - write cycle. Maybe it's just me being
> over cautious.
> 
> Printing a warning might scare users, though. I'd prefer to fix the
> online help, where only "erase and write" is mentioned.

Which document are you referring to? I don't really see the "sf" command
documented anywhere.