USB: serial: ftdi_sio: Add MTP NVM support

[Loic Poulain <loic.poulain@linaro.org>]

Hi Johan, Srini,

On 19 June 2018 at 15:06, Johan Hovold <johan@kernel.org> wrote:
> On Tue, Jun 19, 2018 at 02:32:11PM +0200, Loic Poulain wrote:
>> Hi Johan, Srini,
>>
>> On 18 June 2018 at 11:47, Srinivas Kandagatla
>> <srinivas.kandagatla@linaro.org> wrote:
>> > On 18/06/18 09:46, Johan Hovold wrote:
>
>> >> I'm not necessarily against the idea, but nvmem core needs to be fixed
>> >> so that it can handle hotplugging before this can be considered for
>> >> merging.
>> >>
>> >> Right now it just returns -EBUSY from nvmem_unregister(), which results
>> >> in all kinds of memory leaks, use-after-frees and crashes when user
>> >> space holds the character device open while the device is being
>> >> unplugged.
>>
>> Correct me if I'm wrong, but nvmem is just exposed to userspace as a
>> simple sysfs device attribute (nvmem), removing a device and its attribute(s)
>> dynamically is well managed by sysfs, even if userspace has file open.
>
> My bad, I misremembered what interface nvmem was using and only saw the
> deregistration refusal in nvmem_unregister() during a quick check.
>
>> The only risk here is when a kernel internal consumer still has a reference to
>> the nvmem device on removal, which is not the case in our context.
>
> Right.
>
>> > I can also suggest you to try devm_nvmem_register().
>>
>> Yes, I'm going to send a v2.
>
> I'll take a closer look soonish too.

I changed my mind here, indeed, in order to avoid any potential
use-after-free, we have to manually unregister/release the nvmem
device before freeing of the private port structure (used as nvmem
context). So I will not send a V2 with nvmem devm usage. You can
consider this V1 for review.

Regards,
Loic
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