No more new fbdev drivers, please

[Bernie Thompson]

On Sat, Sep 26, 2015 at 11:01 AM, Geert Uytterhoeven 
 wrote:
> The smallest of these (udl) still counts in at ca. 2800 LoC,

Note udlfb.c, the original fbdev driver that I helped write and that the
udl DRM driver was based on, is ~1800 LoC ... so we're actually talking in
the ballpark of 2x (rather than 10x) between fbdev and DRM in this case.
That said, the complexity difference is probably higher than the LoC
difference. I know I personally have struggled in the shift from
understanding fbdev to understanding DRM.

The fact that there's drivers of both types and USB hardware might make udl
may be a good driver to use as a base for any additional simplification /
helper work. David Airlie and David Herrmann both have this hardware. David
Airlie did the port from fbdev to DRM, so he's made it an exemplary
driver.  And if anyone needs any hardware which works with udlfb and udl,
we're happy to send free hardware to any programmers who are willing to
contribute in the form of code or testing:
http://plugable.com/projects/plugable-open-source-hardware-samples-program

More simplification and documentation would be great. In particular, the
optimization for the connector+encoder+crtc combination others have
mentioned seems like it would be worthwhile.

Cheers,
Bernie

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[UDL] general protection fault in fb_deferred_io_mkwrite()

[Bernie Thompson]

On Sun, Aug 12, 2012 at 3:34 AM, Thomas Meyer  wrote:

> guilty driver is probably udl_fb.c
> any ideas?
>

Hi Thomas,

We were seeing similar issues in udlfb (the original fbdev version of this
driver), which were fixed earlier this year by getting all rendering
operations out of probe/disconnect -- those which might trigger fb_defio
page faults in an inappropriate context, or be long-running. Here's some
more detail:
http://plugable.com/2012/06/21/displaylink-usb-devices-on-linux-kernel-3-4-0/comment-page-1/#comment-5896


Unfortunately, I haven't had time to get going with udl myself, so haven't
been able to port and confirm.  Thanks for raising and staying on this.

Best wishes,
Bernie

[   45.66] RIP  [] fb_deferred_io_mkwrite+0xdc/0xf0
> [   45.66]  RSP 
> [   45.711547] ---[ end trace d4732d5a0bf375fb ]---
> [   45.720961] released /dev/fb1 user=1 count=0
>
>
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Re: [UDL] general protection fault in fb_deferred_io_mkwrite()

[Bernie Thompson]

On Sun, Aug 12, 2012 at 3:34 AM, Thomas Meyer tho...@m3y3r.de wrote:

 guilty driver is probably udl_fb.c
 any ideas?


Hi Thomas,

We were seeing similar issues in udlfb (the original fbdev version of this
driver), which were fixed earlier this year by getting all rendering
operations out of probe/disconnect -- those which might trigger fb_defio
page faults in an inappropriate context, or be long-running. Here's some
more detail:
http://plugable.com/2012/06/21/displaylink-usb-devices-on-linux-kernel-3-4-0/comment-page-1/#comment-5896


Unfortunately, I haven't had time to get going with udl myself, so haven't
been able to port and confirm.  Thanks for raising and staying on this.

Best wishes,
Bernie

[   45.66] RIP  [8123becc] fb_deferred_io_mkwrite+0xdc/0xf0
 [   45.66]  RSP 880126559c98
 [   45.711547] ---[ end trace d4732d5a0bf375fb ]---
 [   45.720961] released /dev/fb1 user=1 count=0


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