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SubjectRe: [PATCH v9 2/4] [media] cxusb: implement Medion MD95700 digital / analog coexistence
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On 24.04.2019 14:45, Hans Verkuil wrote:
> On 4/16/19 1:38 AM, Maciej S. Szmigiero wrote:
>> On 12.04.2019 10:56, Hans Verkuil wrote:
>>> On 3/30/19 12:51 AM, Maciej S. Szmigiero wrote:
>>>> This patch prepares cxusb driver for supporting the analog part of
>>>> Medion 95700 (previously only the digital - DVB - mode was supported).
>>>>
>>>> Specifically, it adds support for:
>>>> * switching the device between analog and digital modes of operation,
>>>> * enforcing that only one mode is active at the same time due to hardware
>>>> limitations.
>>>>
>>>> Actual implementation of the analog mode will be provided by the next
>>>> commit.
>>>>
>>>> Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
>>>> ---
(..)
>>>> index 8056053c9ab0..01987ec5e0c5 100644
>>>> --- a/drivers/media/usb/dvb-usb/dvb-usb-dvb.c
>>>> +++ b/drivers/media/usb/dvb-usb/dvb-usb-dvb.c
>>>> @@ -15,6 +15,7 @@ static int dvb_usb_ctrl_feed(struct dvb_demux_feed *dvbdmxfeed, int onoff)
>>>> {
>>>> struct dvb_usb_adapter *adap = dvbdmxfeed->demux->priv;
>>>> int newfeedcount, ret;
>>>> + bool streaming_ctrl_no_urb;
>>>>
>>>> if (adap == NULL)
>>>> return -ENODEV;
>>>> @@ -24,12 +25,16 @@ static int dvb_usb_ctrl_feed(struct dvb_demux_feed *dvbdmxfeed, int onoff)
>>>> return -EINVAL;
>>>> }
>>>>
>>>> + streaming_ctrl_no_urb = adap->props.fe[adap->active_fe].caps &
>>>> + DVB_USB_ADAP_STREAMING_CTRL_NO_URB;
>>>> newfeedcount = adap->feedcount + (onoff ? 1 : -1);
>>>>
>>>> /* stop feed before setting a new pid if there will be no pid anymore */
>>>> if (newfeedcount == 0) {
>>>> deb_ts("stop feeding\n");
>>>> - usb_urb_kill(&adap->fe_adap[adap->active_fe].stream);
>>>> +
>>>> + if (streaming_ctrl_no_urb)
>>>
>>> Is this test right? Shouldn't it be !streaming_ctrl_no_urb in order to keep the
>>> current (non-medion) behavior?
>>>
>>>> + usb_urb_kill(&adap->fe_adap[adap->active_fe].stream);
>>>>
>>>> if (adap->props.fe[adap->active_fe].streaming_ctrl != NULL) {
>>>> ret = adap->props.fe[adap->active_fe].streaming_ctrl(adap, 0);
>>>> @@ -38,6 +43,9 @@ static int dvb_usb_ctrl_feed(struct dvb_demux_feed *dvbdmxfeed, int onoff)
>>>> return ret;
>>>> }
>>>> }
>>>> +
>>>> + if (!streaming_ctrl_no_urb)
>>>
>>> And then this would have to be inverted as well.
>>
>> The newly added flag "DVB_USB_ADAP_STREAMING_CTRL_NO_URB" has the
>> following meaning:
>> If it is set the order of operations in dvb_usb_ctrl_feed() looks like
>> this:
>> 1) Call the driver streaming_ctrl(1) callback to enable streaming,
>> 2) Submit DVB data URBs,
>> (streaming happens)
>> 3) Kill DVB data URBs,
>> 4) Call the driver streaming_ctrl(0) callback to disable streaming.
>>
>> This is needed for Medion because:
>> a) The device could already be open in the analog mode when
>> streaming_ctrl(1) tries to acquire it in the digital mode.
>> In this case it is important that no DVB URBs are scheduled before
>> giving the callback chance to report an error,
>>
>> b) The device could get open in the analog mode as soon as
>> streaming_ctrl(0) drops the last digital mode reference to it so all
>> DVB data URB must have already been terminated at this point.
>>
>> If the aforementioned flag is unset, however, the order of operations in
>> dvb_usb_ctrl_feed() looks like this:
>> 1) Submit DVB data URBs,
>> 2) Call the driver streaming_ctrl(1) callback to enable streaming,
>> (streaming happens)
>> 3) Call the driver streaming_ctrl(0) callback to disable streaming,
>> 4) Kill DVB data URBs.
>>
>> Previously, the order was always like this:
>> 1) Submit DVB data URBs,
>> 2) Call the driver streaming_ctrl(1) callback to enable streaming,
>> (streaming happens)
>> 3) Kill DVB data URBs,
>> 4) Call the driver streaming_ctrl(0) callback to disable streaming.
>>
>> You can see that this was asymmetric - streaming_ctrl(1) is called with
>> data URBs already active but they are killed before streaming_ctrl(0)
>> gets called, so switching only the submit part on "STREAMING_CTRL_NO_URB"
>> would make this flag operation only half-correct.
>>
>> I have audited existing drivers that use dvb-usb framework and none of
>> them do anything besides a synchronous USB control transfer or a
>> synchronous USB bulk transfer to an endpoint different from the one that
>> DVB data uses in streaming_ctrl(0) callback.
>> Additionally, streaming_ctrl(1) and streaming_ctrl(0) paths are usually
>> very similar in the driver code, so if streaming_ctrl(1) runs fine with
>> data URBs being already active there is no reason to think
>> streaming_ctrl(0) will have a problem with this.
>
> Is there anything wrong with always doing:
>
> 1) Call the driver streaming_ctrl(1) callback to enable streaming,
> 2) Submit DVB data URBs,
> (streaming happens)
> 3) Kill DVB data URBs,
> 4) Call the driver streaming_ctrl(0) callback to disable streaming.

In principle this should be fine - looking at the existing drivers
doesn't reveal anything in their streaming_ctrl(1) callbacks that could
depend on URBs submission order.

> Regards,
>
> Hans
>

Thanks,
Maciej

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