Message-ID: <20150924021521.GA19359@debianG>
Date:	Wed, 23 Sep 2015 23:15:21 -0300
From:	Gaston Gonzalez <gascoar@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	gregkh@...uxfoundation.org, paul.gortmaker@...driver.com,
	dilekuzulmez@...il.com, gdonald@...il.com,
	cristina.opriceana@...il.com, hamohammed.sa@...il.com,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH]  staging: rtl8192u: ieee80211_rx:  Fix incorrect type in
 assignments

On Fri, Jun 26, 2015 at 01:36:14PM -0300, Gaston Gonzalez wrote:
> On Thu, Jun 25, 2015 at 02:06:44PM +0200, Arnd Bergmann wrote:
> > On Wednesday 24 June 2015 13:34:58 Gaston Gonzalez wrote:
> > > On Tue, Jun 23, 2015 at 12:13:47PM +0200, Arnd Bergmann wrote:
> > > > On Sunday 21 June 2015 19:12:09 Gaston Gonzalez wrote:
> > > > >                 /* WMM spec P.11: The minimum value for AIFSN shall be 2 */
> > > > >                 qos_param->aifs[aci] = (qos_param->aifs[aci] < 2) ? 2:qos_param->aifs[aci];
> > > > >  
> > > > > -               qos_param->cw_min[aci] = ac_params->ecw_min_max & 0x0F;
> > > > > +               qos_param->cw_min[aci] =
> > > > > +                       cpu_to_le16(ac_params->ecw_min_max & 0x0F);
> > > > >  
> > > > > -               qos_param->cw_max[aci] = (ac_params->ecw_min_max & 0xF0) >> 4;
> > > > > +               qos_param->cw_max[aci] =
> > > > > +                       cpu_to_le16((ac_params->ecw_min_max & 0xF0) >> 4);
> > > > >  
> > > > >                 qos_param->flag[aci] =
> > > > >                     (ac_params->aci_aifsn & 0x10) ? 0x01 : 0x00;
> > > > > -               qos_param->tx_op_limit[aci] = le16_to_cpu(ac_params->tx_op_limit);
> > > > > +               qos_param->tx_op_limit[aci] = ac_params->tx_op_limit;
> > > > >         }
> > > > >         return 0;
> > > > 
> > > > This certainly needs a more thorough description of how you determined that
> > > > the byte swaps that you add are in fact required. Did you test it on
> > > > a big-endian machine?
> > > > 
> > > > 	Arnd
> > > 
> > > Hello Arnd,
> > > 
> > > Thank you for reviewing this.
> > > After your email and reviwing this again I'm getting a bit suspicious
> > > myself, but this is what I saw:
> > > 
> > > -- First warning:
> > > 
> > > qos_param->cw_min[aci] is defined as __le16() in ieee80211.h
> > > (ieee80211_qos_parameters structure)
> > > 
> > > ac_params-> ecw_min_max is defined as u8 in ieee80211.h
> > > (ieee80211_qos_ac_parameter structure)
> > > 
> > > So the assignment is: __le16 = u8 & 0x0F;
> > > 
> > > -- Second warning:
> > > 
> > > qos_param->cw_max[aci] is __le16()
> > > ac_params-> ecw_min_max is u8
> > > 
> > > The assignment is: __le16 = (u8 & 0xF0) >> 4;
> > > 
> > > Thus, for the warning 1 and 2, I understand that the result won't be the
> > > same if the machine is big-endian or little-endian, and that's why we
> > > need a cpu_to_le16. Am I missing something?
> > 
> > I think your analysis is right, as long as the __le16 annotation is
> > actually correct. It usually helps to look at the git history to
> > see what the intent of the patch was that introduced the assignment
> > and the patch that introduced the __le16 type. Presumably one of them
> > was incorrect, and it would be good to figure out where it went wrong,
> > and to add a 'Fixes:' tag in your patch description that pinpoints
> > the exact mistake.
> > 
> 
> Ok, will do.
> 
> > > -- Third warning:
> > > 
> > > In this case both sides of the assignment are already defined as __le16:
> > > 
> > > qos_param->tx_op_limit[aci] (ieee80211_qos_parameters structure defined
> > > in ieee80211.h))
> > > 
> > > ac_params->tx_op_limit (ieee80211_qos_ac_parameter structure defined in
> > > ieee80211.h)
> > > 
> > > So the assignment is: __le16() = le16_to_cpu(__le16)
> > > 
> > > Im getting suspicious now, but it sounded wrong to me.
> > > In the case the right part is correct, I guess the left part should be
> > > u16 type?
> > 
> > Again, your logic sounds good: there is clearly something wrong here, but
> > it's not obvious to conclude whether it is an incorrect annotation or
> > an extraneous byte swap. Besides looking at the git history, it also
> > helps to look at all other uses of the two sides of the assignment:
> > 
> > See if qos_param->tx_op_limit is in fact used as a little-endian
> > value (e.g. by copying to memory or a register), and if the value that
> > got written to ac_params->tx_op_limit was byte-swapped already at
> > the time of assignment.
> > 
> Ok, I'll do it too.
> 
> > > Regarding the test: I tested it on my machine, but is of course little-
> > > endian :( I could built a qemu virtual machine to test it on a
> > > big-endian emulated platform. Should that work?
> > 
> > Yes, that would work: you can assign the USB device to the qemu machine
> > and run a kernel in there. The easiest emulation to try is probably
> > a PowerPC PAPR machine with a file system from
> > https://people.debian.org/~aurel32/qemu/powerpc/.
> > MIPS should work as well.
> > 
> 
> Ok, thanks a lot for all the pointers.
> 
> Gaston
> > 	Arnd

So more than two months have passed without any reply from my side.

The thing is I'm struggling to get a new hardware with this chipset. Today I
received my fourth rtl8192cu device to my collection of wrong devices, which
also comprises one ralink and some other chip. All of them specified as
rtl8192u on the sellers site...

I have one or two options left to try to get a replacement rtl8192u device,
but for the moment I don't have the hardware.

Though I know this 'change'is almost insignificant I wanted to give some 
life signal and not leave this thread as abandoned.

regards,

Gaston
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