[mythtv-users] /dev/dvb permissions

John Pilkington johnpilk222 at gmail.com
Mon Jun 28 16:07:41 UTC 2021

On 28/06/2021 06:27, Eyal Lebedinsky wrote:
> On 28/06/2021 14.16, Stephen Worthington wrote:
>> On Mon, 28 Jun 2021 10:50:23 +1000, you wrote:
>>> On 21/06/2021 23.49, Eyal Lebedinsky wrote:
>>>> After a recent update (fedora 34) mythbackend failed to show any card.
>>>> Short story:
>>>>   ����crw-rw----+ 1 root rtlsdr 212,� 3 Jun 20 23:10 
>>>> /dev/dvb/adapter0/frontend0
>>>> so user mythtv cannot open it. chmod fixed it, and I added this to 
>>>> my startup scripts.
>>>> Was this an intended change? This update brought in
>>>>   ����mythtv-backend-31.0-17.147.20210421git05c16580e1.fc34.x86_64
>>>> and friends as well as
>>>>   ����kernel-5.12.11-300.fc34.x86_64
>>>>   ����rtl-sdr-0.6.0-9.fc34.x86_64
>>>> The group rtlsdr is new:
>>>>   ����Jun 20 22:43:17 e7 groupadd[1678612]: group added to 
>>>> /etc/group: name=rtlsdr, GID=440
>>>>   ����Jun 20 22:43:17 e7 groupadd[1678612]: group added to 
>>>> /etc/gshadow: name=rtlsdr
>>>>   ����Jun 20 22:43:17 e7 groupadd[1678612]: new group: name=rtlsdr, 
>>>> GID=440
>>>> Is there a possible conflict?
>>>> Should user mythtv be added to the new group?
>>> I still do not know what the correct action should be.
>>> 1) I see
>>>     /usr/lib/udev/rules.d/10-rtl-sdr.rules
>>> is setting the group rtlsdr to a list of devices which includes my 
>>> tuners 0bda:2838
>>> Is this proper to do so when there are other users of these devices?
>>> 2) If root.rtlsdr is correct, should the user 'mythtv' also be a 
>>> member of 'rtlsdr'?
>>>     How should this be done without every mythtv user doing it 
>>> manually once the backend fails?
>>>     Is this an issue to be dealt with by the rtl-sdr folks?
>>> I suspect that the rtl-sdr package caused the new group to be applied 
>>> to the devices without
>>> consideration for other users of these devices.
>>> I will raise this conflict on the fedora list, however mythtv is from 
>>> @rpmfusion and rtl-sdr is from @fedora.
>> This is a conflict between the rtl-sdr package and the MythTV
>> packages.  If you are not using the SDR software, you should just
>> remove the SDR packages.  If you do want to use the SDR software, you
>> will need to work out some arrangement that gives MythTV access to the
>> tuners.  Adding the frontend user and backend user ("mythtv") to the
>> "rtlsdr" group is an option that should work.
> This is my plan B.
>> Another option is to do
>> chmod a+rw on all the tuner devices.
> Done that, and added the chmod command to rc.local too.
>> However, MythTV does need
>> exclusive access to the tuners when it is using them.  So if the SDR
>> software is using them when MythTV wants to, MythTV will fail to
>> access the tuners.  And in MythTV you have to set the option on each
>> tuner that makes MythTV release access to it when it is not using it.
> Sure. I plan to dedicate one tuner (there are 5) to rtl-sdr if I use it.
>> And you need to turn off EIT use of the tuners, otherwise MythTV will
>> use them for EIT EPG grabbing when it is not using them for recording.
> EIT is already set to run on only one tuner.
>> It would be much easier just to remove the SDR packages.
> True, and should be trivial to install (with due care) when needed.
> However, the general solution should be something that fedora users are 
> unlikely to trip over.
> Right now installing rtl-sdr will make mythbackend fail which may not be 
> trivial to diagnose
> (took me a while to realize what happened).
> cheers
I see this is also on the Fedora lists.

I'm running mythtv master with dvb tuners in Fedora 33.  No similar 
problem seen.  rtl-sdr is installed, but I don't have its target device. 
Perhaps you do?

rtl-sdr is 'SDR utilities for Realtek RTL2832 based DVB-T dongles' which 
can 'turn your  ... dongle into an SDR receiver'

John P

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