Re: [Ilw] Question about iwl_pcie_rxq_inc_wr_ptr

[Stanislaw Gruszka <sgruszka@redhat.com>]

On Mon, Aug 19, 2013 at 09:32:58AM +0000, Grumbach, Emmanuel wrote:
> > 
> > I would like to ask if below change is safe (I'm considering to do that change
> > on iwlegacy):
> > 
> > diff --git a/drivers/net/wireless/iwlwifi/pcie/rx.c
> > b/drivers/net/wireless/iwlwifi/pcie/rx.c
> > index 567e67a..1ebdb83 100644
> > --- a/drivers/net/wireless/iwlwifi/pcie/rx.c
> > +++ b/drivers/net/wireless/iwlwifi/pcie/rx.c
> > @@ -183,7 +183,7 @@ static void iwl_pcie_rxq_inc_wr_ptr(struct iwl_trans
> > *trans, struct iwl_rxq *q)
> >  		} else {
> >  			/* Device expects a multiple of 8 */
> >  			q->write_actual = (q->write & ~0x7);
> > -			iwl_write_direct32(trans, FH_RSCSR_CHNL0_WPTR,
> > +			iwl_write32(trans, FH_RSCSR_CHNL0_WPTR,
> >  				q->write_actual);
> >  		}
> >  	}
> > 
> > This register seems to be only read by firmware, so maybe we can modify it
> > without grab nic access. We are doing that on iwl_pcie_txq_inc_wr_ptr().
> > 
> 
> I am not sure...
> I am not sure what would be the behavior: not updating the value at all, or having the HW use an old value for a while.
> Does this cost you so much CPU that it justifies such a change?

I have those bug reports:

https://bugzilla.redhat.com/show_bug.cgi?id=863386
https://bugzilla.redhat.com/show_bug.cgi?id=889467
https://bugzilla.redhat.com/show_bug.cgi?id=895650
https://bugzilla.redhat.com/show_bug.cgi?id=989025

All are 4965 specific - I haven't seen similar traces on iwlwifi. On
some cases use nohz=off boot option helped with the problem, so this
could be also some kernel issue when udelay() is not working as
expected. Anyway I'm considering to remove grab_nic_access from that
path and would like to know possible consequences from firmware
perspective.

Stanislaw