RE: [PATCH V2 2/5] can: flexcan: add CAN FD mode support

[Joakim Zhang]

Hi Stefan,

Thanks for your validation! Could you add your test tag if you can successfully validated?

Comments in-lined about your points.

Best Regards,
Joakim Zhang

> -----Original Message-----
> From: Stefan-gabriel Mirea
> Sent: 2019年4月9日 1:46
> To: Joakim Zhang <qiangqing.zhang@xxxxxxx>; linux-can@xxxxxxxxxxxxxxx;
> netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH V2 2/5] can: flexcan: add CAN FD mode support
> 
> Hello Joakim,
> 
> I successfully validated your CAN FD support on an S32V234 based board.
> Here are some points which are not very clear to me:
> 
> Joakim Zhang <qiangqing.zhang@xxxxxxx> writes:
> > +		priv->write(reg_mcr | FLEXCAN_MCR_FDEN, &regs->mcr);
> 
> According to the S32V234 reference manual (not sure if this applies to
> i.MX8QM as well), CAN_CTRL1[SMP] cannot be asserted when CAN FD is
> enabled. Is it safe to configure the interface with both "fd on" and
> "triple-sampling on"? If not, flexcan_open should return an error when both
> CAN_CTRLMODE_FD and CAN_CTRLMODE_3_SAMPLES are set in ctrlmode.
 
 [Joakim Zhang] Yes, you are right. I checked in i.MX8QM reference manual, CAN_CTRLMODE_FD and CAN_CTRLMODE_3_SAMPLES should not set together. I will fix it in V3.

> Joakim Zhang <qiangqing.zhang@xxxxxxx> writes:
> > +		priv->offload.is_canfd = true;
> 
> Shouldn't is_canfd be assigned false in the "else" branch? Otherwise, is_canfd
> will stay true after the following sequence:
> 
> root@s32v234evb:~# ip link set can0 type can bitrate 500000 dbitrate
> 2000000 fd on
> root@s32v234evb:~# ip link set can0 up
> root@s32v234evb:~# ip link set can0 down root@s32v234evb:~# ip link set
> can0 type can bitrate 500000 fd off root@s32v234evb:~# ip link set can0 up
 
 [Joakim Zhang] Good catch! I will fix it in V3.

> Moreover, I wonder if we even need is_canfd, because frames should be fed to
> the kernel as classic or FD based only on the EDL bit in the C/S word of the Rx
> MB, to keep their original format. That is, you may either read EDL from within
> a new callback (but I think that mailbox_read should remain the only one
> which both locks and unlocks the
> mailboxes) or let mailbox_read call alloc_can(fd)_skb itself.

 [Joakim Zhang] We added can fd support in 4.9 kernel which let mailbox_read call alloc_can(fd)_skb in the past. Now the driver allocate skb before mailbox_read in rx_offload and also read the overflow frames.
              I add the "is_canfd" since I don't want to change the rx_offload framework too much. @mkl@xxxxxxxxxxxxxx, could you give some advice, which solution is better?
              
> Regards,
> Stefan