netdev - Re: [PATCH] net: qualcomm: rmnet: Allow partial updates of IFLA

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Message-ID: <32488f5d-c493-c8c4-fbf7-4370d6460f44@linaro.org>
Date:   Thu, 22 Apr 2021 20:01:23 -0500
From:   Alex Elder <elder@...aro.org>
To:     subashab@...eaurora.org
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Sean Tranchetti <stranche@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Daniele Palmas <dnlplm@...il.com>,
        Aleksander Morgado <aleksander@...ksander.es>,
        Loic Poulain <loic.poulain@...aro.org>
Subject: Re: [PATCH] net: qualcomm: rmnet: Allow partial updates of IFLA_FLAGS

On 4/22/21 6:28 PM, subashab@...eaurora.org wrote:
> On 2021-04-22 12:29, Alex Elder wrote:
>> On 4/22/21 1:20 PM, Bjorn Andersson wrote:
>>> The idiomatic way to handle the changelink flags/mask pair seems to be
>>> allow partial updates of the driver's link flags. In contrast the rmnet
>>> driver masks the incoming flags and then use that as the new flags.
>>>
>>> Change the rmnet driver to follow the common scheme, before the
>>> introduction of IFLA_RMNET_FLAGS handling in iproute2 et al.
>>
>> I like this a lot.  It should have been implemented this way
>> to begin with; there's not much point to have the mask if
>> it's only applied to the passed-in value.
>>
>> KS, are you aware of *any* existing user space code that
>> would not work correctly if this were accepted?
>>
>> I.e., the way it was (is), the value passed in *assigns*
>> the data format flags.  But with Bjorn's changes, the
>> data format flags would be *updated* (i.e., any bits not
>> set in the mask field would remain with their previous
>> value).
>>
>> Reviewed-by: Alex Elder <elder@...aro.org>
> 
> What rmnet functionality which was broken without this change.
> That doesnt seem to be listed in this patch commit text.

The broken functionality is that RMNet is not using the
value/flag pair in the proper way.

Currently, the RMNet driver assigns the flags value,
and (strangly) applies the mask to that value.

The intent of the value/flag pair interface is to allow
a value to be provided, with a mask of bits that indicate
which bits in the value should be *updated* in the target
field stored in the kernel.

That way, one can *assign* a value (by providing a value
with flag value 0xffffffff), but one can also update one
or any number of bits, preserving existing values.

It means, for example, that a request can preserve
existing settings, while *adding* a receive checksum
offload.

> If this is an enhancement, then patch needs to be targeted to net-next
> instead of net

Bjorn targeted neither net nor net-next.  He just posted
the patch.  I think it could be either.

					-Alex