Re: [PATCH] net: 6pack: Fix tx timeout and slot time

[Guenter Roeck]

On 9/9/21 8:27 AM, David Miller wrote:
> From: Guenter Roeck <linux@xxxxxxxxxxxx>
> Date: Thu, 9 Sep 2021 07:53:29 -0700
>
> > On 9/9/21 4:34 AM, David Miller wrote:
> > > From: Guenter Roeck <linux@xxxxxxxxxxxx>
> > > Date: Wed,  8 Sep 2021 20:57:43 -0700
> > >
> > > > tx timeout and slot time are currently specified in units of HZ.
> > > > On Alpha, HZ is defined as 1024. When building alpha:allmodconfig,
> > > > this results in the following error message.
> > > >
> > > > drivers/net/hamradio/6pack.c: In function 'sixpack_open':
> > > > drivers/net/hamradio/6pack.c:71:41: error:
> > > > 	unsigned conversion from 'int' to 'unsigned char'
> > > > 	changes value from '256' to '0'
> > > >
> > > > In the 6PACK protocol, tx timeout is specified in units of 10 ms
> > > > and transmitted over the wire. Defining a value dependent on HZ
> > > > doesn't really make sense. Assume that the intent was to set tx
> > > > timeout and slot time based on a HZ value of 100 and use constants
> > > > instead of values depending on HZ for SIXP_TXDELAY and SIXP_SLOTTIME.
> > > >
> > > > Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> > > > ---
> > > > No idea if this is correct or even makes sense. Compile tested only.
> > > These are timer offsets so they have to me HZ based.  Better to make
> > > the
> > > structure members unsigned long, I think.
> >
> > Hmm, ok. Both tx_delay and slottime are updated in sp_encaps(),
> > though,
> > from data in the transmit buffer. The KISS protocol description states
> > that the values are in units of 10ms; that is where my assumption
> > came from.
>
> They are ms and must be converted using to HZ in order to use the values as timer offsets.
>
> The values are perfectly fine, the types used to store them need to be fixed.
>
> > Anyway, I am inclined to just mark the protocol as dependent on
> > !ALPHA. Would you accept that ?
>
> No, fix this properly.  Make the unsigfned char members be unsigned long.
>
> Why do you not want to fix it this way?

All I want is to get alpha:allmodconfig to compile, nothing else,
but at the same time I don't want to introduce new bugs.

If I make tx_delay unsigned long, it is still passed to encode_sixpack()
as parameter, and that parameter is declared unsigned char and put into
a byte sized buffer. If the value is 0x100, it will then be truncated to
0x0, only that is then done silently without generating a compile error.
Sure, that fixes the compile error, but not the underlying issue. Maybe
that truncation is perfectly fine, but I don't understand the code well
enough to make that call.

Never mind, I'll let someone else who has a better understanding of the code
deal with this.

Guenter