linux-kernel - Re: [PATCH 1/8] net: qualcomm: rmnet: fix struct rmnet_map

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Date:   Tue, 21 May 2019 06:03:58 -0500
From:   Alex Elder <elder@...aro.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        arnd@...db.de, david.brown@...aro.org, agross@...nel.org,
        davem@...emloft.net, ilias.apalodimas@...aro.org,
        cpratapa@...eaurora.org, syadagir@...eaurora.org,
        evgreen@...omium.org, benchan@...gle.com, ejcaruso@...gle.com,
        netdev@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/8] net: qualcomm: rmnet: fix struct rmnet_map_header

On 5/20/19 10:07 PM, Bjorn Andersson wrote:
> On Mon 20 May 19:30 PDT 2019, Alex Elder wrote:
> 
>> On 5/20/19 8:32 PM, Subash Abhinov Kasiviswanathan wrote:
>>>>
>>>> If you are telling me that the command/data flag resides at bit
>>>> 7 of the first byte, I will update the field masks in a later
>>>> patch in this series to reflect that.
>>>>
>>>
>>> Higher order bit is Command / Data.
>>
>> So what this means is that to get the command/data bit we use:
>>
>> 	first_byte & 0x80
>>
>> If that is correct I will remove this patch from the series and
>> will update the subsequent patches so bit 7 is the command bit,
>> bit 6 is reserved, and bits 0-5 are the pad length.
>>
>> I will post a v2 of the series with these changes, and will
>> incorporate Bjorn's "Reviewed-by".
>>
> 
> But didn't you say that your testing show that the current bit order is
> wrong?

I did say that, but it seems I may have been misinterpreting
what the documentation said, namely that "bit 0" in the network
data stream is actually the high-order bit in the first byte.

I did definitely see that bit 7 (0x80) in the first byte was the
one selected by the "cd_bit" C bit-field originally, and I believed
that was wrong.

The other thing I can say is that I never see that bit set in my
use of the rmnet driver for IPA.  On top of that, the pad_len
value is 0.  Given that, either bit order works, because the
whole first byte is 0 either way.  So it turns out the testing
I am able to do is not adequate to verify the change.

I am hoping that Subash has an environment in which QMAP
commands (with the appropriate bit set) are actually used.

I'm going to wait a bit for him to confirm that, but at this
time my plan is to do as I said above--remove this patch and
adjust the ones that follow accordingly.

					-Alex

> I still like the cleanup, if nothing else just to clarify and clearly
> document the actual content of this header.
> 
> Regards,
> Bjorn
>