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Date: Mon, 22 Mar 2021 16:33:29 +0100
From: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
To: Marc Gonzalez <marc.w.gonzalez@...e.fr>
Cc: linux-media <linux-media@...r.kernel.org>,
Brad Love <brad@...tdimension.cc>, Sean Young <sean@...s.org>,
Arnd Bergmann <arnd@...db.de>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 4/4] media: dvb_frontend: disable zigzag mode if not
possible
Em Thu, 18 Jun 2020 11:50:54 +0200
Marc Gonzalez <marc.w.gonzalez@...e.fr> escreveu:
> On 17/06/2020 20:52, Mauro Carvalho Chehab wrote:
>
> > For the zigzag to work, the core needs to have a frequency
> > shift. Without that, the zigzag code will just try re-tuning
> > several times at the very same frequency, with seems wrong.
>
> s/with/which
>
> Suggest: "the core requires a frequency shift value"
>
> > So, add a warning when this happens, and fall back to the
> > single-shot mode.
> >
> > Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
> > ---
> > drivers/media/dvb-core/dvb_frontend.c | 141 +++++++++++++++-----------
> > 1 file changed, 79 insertions(+), 62 deletions(-)
>
> It's hard to discern in the diff what is just white-space adjustment
> from one less tab, and what is new code that requires more scrutiny.
> I'll try applying the patch, and then diff -w.
> Yes, that's much better.
>
> > diff --git a/drivers/media/dvb-core/dvb_frontend.c b/drivers/media/dvb-core/dvb_frontend.c
> > index ed85dc2a9183..cb577924121e 100644
> > --- a/drivers/media/dvb-core/dvb_frontend.c
> > +++ b/drivers/media/dvb-core/dvb_frontend.c
> > @@ -642,6 +642,9 @@ static void dvb_frontend_wakeup(struct dvb_frontend *fe)
> > wake_up_interruptible(&fepriv->wait_queue);
> > }
> >
> > +static u32 dvb_frontend_get_stepsize(struct dvb_frontend *fe);
> > +static void prepare_tuning_algo_parameters(struct dvb_frontend *fe);
> > +
> > static int dvb_frontend_thread(void *data)
> > {
> > struct dvb_frontend *fe = data;
> > @@ -696,78 +699,92 @@ static int dvb_frontend_thread(void *data)
> > fepriv->reinitialise = 0;
> > }
> >
> > - /* do an iteration of the tuning loop */
> > - if (fe->ops.get_frontend_algo) {
> > + if (fe->ops.get_frontend_algo)
> > algo = fe->ops.get_frontend_algo(fe);
> > - switch (algo) {
> > - case DVBFE_ALGO_HW:
> > - dev_dbg(fe->dvb->device, "%s: Frontend ALGO = DVBFE_ALGO_HW\n", __func__);
> > + else
> > + algo = DVBFE_ALGO_SW;
> >
> > - if (fepriv->state & FESTATE_RETUNE) {
> > - dev_dbg(fe->dvb->device, "%s: Retune requested, FESTATE_RETUNE\n", __func__);
> > - re_tune = true;
> > - fepriv->state = FESTATE_TUNED;
> > - } else {
> > - re_tune = false;
> > - }
> > + /* do an iteration of the tuning loop */
> > + switch (algo) {
> > + case DVBFE_ALGO_SW:
> > + prepare_tuning_algo_parameters(fe);
> >
> > - if (fe->ops.tune)
> > - fe->ops.tune(fe, re_tune, fepriv->tune_mode_flags, &fepriv->delay, &s);
> > -
> > - if (s != fepriv->status && !(fepriv->tune_mode_flags & FE_TUNE_MODE_ONESHOT)) {
> > - dev_dbg(fe->dvb->device, "%s: state changed, adding current state\n", __func__);
> > - dvb_frontend_add_event(fe, s);
> > - fepriv->status = s;
> > - }
> > - break;
> > - case DVBFE_ALGO_SW:
> > + if (fepriv->max_drift) {
> > dev_dbg(fe->dvb->device, "%s: Frontend ALGO = DVBFE_ALGO_SW\n", __func__);
> > dvb_frontend_swzigzag(fe);
> > break;
> > - case DVBFE_ALGO_CUSTOM:
> > - dev_dbg(fe->dvb->device, "%s: Frontend ALGO = DVBFE_ALGO_CUSTOM, state=%d\n", __func__, fepriv->state);
> > - if (fepriv->state & FESTATE_RETUNE) {
> > - dev_dbg(fe->dvb->device, "%s: Retune requested, FESTAT_RETUNE\n", __func__);
> > - fepriv->state = FESTATE_TUNED;
> > + }
> > +
> > + /*
> > + * See prepare_tuning_algo_parameters():
> > + * - Some standards may not use zigzag.
> > + */
> > + if (!dvb_frontend_get_stepsize(fe))
> > + dev_warn(fe->dvb->device,
> > + "disabling sigzag, as frontend doesn't set frequency step size\n");
>
> s/sigzag/zigzag
>
> I don't understand why you're calling dvb_frontend_get_stepsize() again?
> prepare_tuning_algo_parameters() already tried its best to set fepriv->step_size
>
> Why not just:
>
> if (fepriv->max_drift)
> do the zigzag
> else
> warn that zigzag is disabled
>
> > +
> > + /* fall through */
>
> Why would you want to fall through from DVBFE_ALGO_SW to DVBFE_ALGO_HW?
> I think this changes the behavior before the patch.
I double-checked this patch. What happens is that there are 3
types of DVB devices:
1. Devices where the Zigzag happens at the hardware level,
automatically (DVBFE_ALGO_HW). All they need is to call
fe->ops.tune() logic once;
2. Devices that have their own hardware-assisted zigzag logic.
Those are handled via DVBFE_ALGO_CUSTOM logic. Those use
an special callback: fe->ops.search(fe).
3. Devices that require the Kernel to do zigzag (DVBFE_ALGO_SW).
Those should set max_drift and other fields, in order to
setup the zigzag steps.
In other words, a device driver which uses DVBFE_ALGO_SW should
provide the vars that are needed for the zigzag to work, as
otherwise, the software zigzag would be just wasting time, as
it won't be different than a device driver using DVBFE_ALGO_HW.
What the above patch does is to generate a warning when
DVBFE_ALGO_SW is used without setting the frequency shift,
which is an uAPI/kAPI violation. On such cases, it will fallback
to DVBFE_ALGO_HW.
The main issue is that testing this patch is not trivial.
As you pointed, it can cause regressions. So, instead of this
patch, I'll merge one that will just print a warning. We need
to fix the frontend drivers case by case.
Thanks,
Mauro
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