linux-kernel - Re: [PATCH] staging: rtl8192u: ieee80211_rx: Fix incorrect type in assignments

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Message-ID: <20150624163458.GA1613@debsktop1>
Date:	Wed, 24 Jun 2015 13:34:58 -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,
	gascoar@...il.com
Subject: Re: [PATCH]  staging: rtl8192u: ieee80211_rx:  Fix incorrect type in
 assignments

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?

-- 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?

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?

Regards,

Gaston
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