linux-kernel - Re: [PATCH] lightnvm: move device L2P detection to core

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Date:   Fri, 3 Aug 2018 13:19:15 +0000
From:   Javier Gonzalez <javier@...xlabs.com>
To:     Matias Bjørling <mb@...htnvm.io>
CC:     "Konopko, Igor J" <igor.j.konopko@...el.com>,
        "marcin.dziegielewski@...el.com" <marcin.dziegielewski@...el.com>,
        Hans Holmberg <hans.holmberg@...xlabs.com>,
        Heiner Litz <hlitz@...c.edu>,
        Young Tack Tack Jin <youngtack.jin@...cuitblvd.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lightnvm: move device L2P detection to core

> On 3 Aug 2018, at 15.01, Matias Bjørling <mb@...htnvm.io> wrote:
> 
> On 08/03/2018 02:55 PM, Javier Gonzalez wrote:
>>> On 3 Aug 2018, at 14.43, Matias Bjørling <mb@...htnvm.io> wrote:
>>> 
>>> On 08/03/2018 02:40 PM, Javier Gonzalez wrote:
>>>>> On 3 Aug 2018, at 14.37, Matias Bjørling <mb@...htnvm.io> wrote:
>>>>> 
>>>>> On 08/03/2018 02:16 PM, Javier Gonzalez wrote:
>>>>>>> On 3 Aug 2018, at 10.54, Matias Bjørling <mb@...htnvm.io> wrote:
>>>>>>> 
>>>>>>> A 1.2 device is able to manage the logical to physical mapping
>>>>>>> table internally or leave it to the host.
>>>>>>> 
>>>>>>> A target only supports one of those approaches, and therefore must
>>>>>>> check on initialization. Move this check to core to avoid each target
>>>>>>> implement the check.
>>>>>>> 
>>>>>>> Signed-off-by: Matias Bjørling <mb@...htnvm.io>
>>>>>>> ---
>>>>>> I see where you want to go with these changes, but the way targets are
>>>>>> layered on top of the LightNVM subsystem does not align with it.
>>>>>> LightNVM can support different OCSSD versions and capabilities, but that
>>>>>> does not mean that a target (e.g., pblk) does. The way I see it, core
>>>>>> should only check for (i) the drive to expose itself in a known revision
>>>>>> and (ii) the reported structures to be consistent. However, specific
>>>>>> functionality is not for core to check upo.
>>>>> 
>>>>> Why try to initialize a target, if we already know that it is incompatible?
>>>> Yes, that is my point. But the one who knows if the targets supports
>>>> something or not is the target, not the subsystem. Here, you are making
>>>> an assumption knowing the pblk requires the L2P on the host, but that
>>>> could change in the future...
>>> 
>>> I don't believe it can. It is not supported by the 2.0 specification.
>>> 1.2 is legacy.
>> Ja... We both know that people is using 1.2 variants out there...
> 
> Yes, I'm not saying that. I'm saying the spec 1.2 has been deprecated
> by 2.0 and no longer developed,

But we need to maintain it either way.

> and I'm not going to move that brainfart into the 2.0 spec :)

I would not ask you to. Luckily, no-one has asked for it yet...

>>> I understand this from the perspective when checking for un-even
>>> configurations using the geometry. But this is a spec incompatibility,
>>> which I don't think the target should care about.
>> I see the point of not having this check in pblk since we know that we
>> are moving towards 2.0 and leaving 1.2 as legacy/not-upstream. But does
>> it really make sense to fail LightNVM on a 1.2 capability that is spec.
>> compliant? For all we know people could have this and use it from user
>> space or through an internal target.
> 
> It only fails on target creation, not on disk initialization. The disk
> will be up so user-space and targets that implements. Is that a
> problem?

My only point is that there are different responsibilities for core and
targets and some things are supported by 1.2 and not by 2.0, but that is
ok. If someone wants to create a hybrid 1.2 device on top of LightNVN -
in my view -, they should be allowed to.

Anyway, this is more a general layering discussion than actual
opposition to the patch itself.

Javier

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