netdev - Re: [v8, bpf-next, 4/9] net/wireless/iwlwifi: fix iwlwifi_dev_ucode

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Date:   Thu, 24 May 2018 16:28:39 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Alexei Starovoitov <ast@...nel.org>, davem@...emloft.net,
        daniel@...earbox.net, torvalds@...ux-foundation.org,
        peterz@...radead.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, netdev@...r.kernel.org,
        kernel-team@...com, linux-api@...r.kernel.org,
        linux-wireless@...r.kernel.org
Subject: Re: [v8, bpf-next, 4/9] net/wireless/iwlwifi: fix
 iwlwifi_dev_ucode_error tracepoint

On Wed, May 23, 2018 at 01:03:08PM +0200, Johannes Berg wrote:
> On Wed, 2018-03-28 at 12:05 -0700, Alexei Starovoitov wrote:
> > fix iwlwifi_dev_ucode_error tracepoint to pass pointer to a table
> > instead of all 17 arguments by value.
> > dvm/main.c and mvm/utils.c have 'struct iwl_error_event_table'
> > defined with very similar yet subtly different fields and offsets.
> > tracepoint is still common and using definition of 'struct iwl_error_event_table'
> > from dvm/commands.h while copying fields.
> > Long term this tracepoint probably should be split into two.
> 
> It would've been nice to CC the wireless list for wireless related
> patches ...

Ohh. I didn't realize that networking wireless doesn't fall under netdev.
I thought wireless folks are silent because they are embarrassed
by a function with 17 arguments.

> > --- a/drivers/net/wireless/intel/iwlwifi/iwl-devtrace.c
> > +++ b/drivers/net/wireless/intel/iwlwifi/iwl-devtrace.c
> > @@ -30,6 +30,7 @@
> >  #ifndef __CHECKER__
> >  #include "iwl-trans.h"
> >  
> > +#include "dvm/commands.h"
> 
> In particular, this breaks the whole driver abstraction.
> 
> > +++ b/drivers/net/wireless/intel/iwlwifi/mvm/utils.c
> > @@ -549,12 +549,7 @@ static void iwl_mvm_dump_lmac_error_log(struct iwl_mvm *mvm, u32 base)
> >  
> >         IWL_ERR(mvm, "Loaded firmware version: %s\n", mvm->fw->fw_version);
> >  
> > -       trace_iwlwifi_dev_ucode_error(trans->dev, table.error_id, table.tsf_low,
> > -                                     table.data1, table.data2, table.data3,
> > -                                     table.blink2, table.ilink1,
> > -                                     table.ilink2, table.bcon_time, table.gp1,
> > -                                     table.gp2, table.fw_rev_type, table.major,
> > -                                     table.minor, table.hw_ver, table.brd_ver);
> > +       trace_iwlwifi_dev_ucode_error(trans->dev, &table, table.hw_ver, table.brd_ver);
> 
> This is also utterly wrong because mvm has - for better or worse - a
> different type "struct iwl_error_event_table" in this file ...

As I was trying to explain in the commit log the single struct
is used in both places, but differences in two
"struct iwl_error_event_table" are carefully matched
field and by field. For two extra fields it was not
possible and they are passed separately as you can see above.
I still believe that tracepoint output is still exactly
the same before and after the patch.
I guess you see the breakage because new fields got
added into one "struct iwl_error_event_table",
but were not added to its evil twin "struct iwl_error_event_table"
with the same name after the patch landed ?
imo wireless folks need to avoid such naming conflicts.
I suggest to isolate common fields into separate base struct and
give two children structs different names.