USB: serial: ftdi_sio: Add MTP NVM support

Hi Johan, Srini,

On 19 June 2018 at 15:06, Johan Hovold [off-list ref] wrote:

On Tue, Jun 19, 2018 at 02:32:11PM +0200, Loic Poulain wrote:

quoted

Hi Johan, Srini,

On 18 June 2018 at 11:47, Srinivas Kandagatla
[off-list ref] wrote:

quoted

On 18/06/18 09:46, Johan Hovold wrote:

quoted

quoted

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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.

quoted

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.

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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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