Re: [PATCH 1/8] net: qualcomm: rmnet: fix struct rmnet_map_header

From: Alex Elder <elder@linaro.org>
To: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>,
	arnd@arndb.de, david.brown@linaro.org, agross@kernel.org,
	davem@davemloft.net, ilias.apalodimas@linaro.org,
	cpratapa@codeaurora.org, syadagir@codeaurora.org,
	evgreen@chromium.org, benchan@google.com, ejcaruso@google.com,
	netdev@vger.kernel.org, linux-arm-msm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/8] net: qualcomm: rmnet: fix struct rmnet_map_header
Date: Tue, 21 May 2019 06:03:58 -0500	[thread overview]
Message-ID: <25b1d768-d492-08a7-b1ab-d3d022b01bc9@linaro.org> (raw)
In-Reply-To: <20190521030712.GY2085@tuxbook-pro>

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
> 