Custom KMS driver + X

[Jose Abreu]

Hi Adam,

Thanks for your answer!


On 06-07-2016 15:57, Adam Jackson wrote:
> On Tue, 2016-07-05 at 15:28 +0100, Jose Abreu wrote:
>
>>      - First: My driver only supports 24 bpp (I mean real 24
>> bits, not 24 packed in 32 bits). Is there a way to specify to X
>> (or specify in the driver itself) to use 24bpp **only** in the
>> new driver that I created?
> Kind of. The arguments to xf86SetDepthBpp (which you're probably
> calling in your PreInit hook) specify how the root window is to be
> treated. If you can only do 24bpp, you'll want to pass Support24bppFb
> (and nothing else) as the last argument. By default (ie, without exa or
> glamor or other acceleration) other pixmaps will be created in host
> memory as 32bpp, and the software renderer will convert between formats
> as needed. If you want to add support for pixmaps in video memory
> you'll need to enforce the 24bpp restriction in your accel support
> code.

Ok, something is missing because I am not calling
xf86SetDetpthBpp at all. You mean I need to have a X driver (or
module, I don't know the nomenclature) in order to use my KMS
driver? Can't I just use the 'modesetting' driver? Sorry if I am
making some dumb mistake but my understanding of the inner
workings of X is barely null.

Right now I am able to force X to use 24bpp in the KMS driver. I
just return -EINVAL in the dumb_create() callback of DRM when the
bpp is not 24 and then X tries again with the right bpp, but when
I do this the X stops working and in the log it appears the stack
trace that I sent in the first email.

>
> That said, a lot of client software these days gets so confused by
> 24bpp that we've added code to the stock modesetting driver to maintain
> a 32bpp shadow framebuffer in host memory, and convert at screen update
> time:
>
> https://cgit.freedesktop.org/xorg/xserver/commit/?id=21217d02168d1883b2d1f64399aec494f96a8b9d
>
> If you can possibly support 32bpp instead I recommend it. If you can't
> but aren't doing any hardware accel, the 32→24 conversion approach
> above is probably best. If you really do want 24bpp with hardware
> accel, well, have fun.
>
>>     - Second (and most important one): If I start my workstation
>> and modprobe the driver the X server seems to recognize and add
>> the new DRM device (I can't be sure because right now no output
>> is shown due to the 24/32 bpp problem). But if I then do a
>> "startx" from terminal the server will crash with a segmentation
>> fault, outputting this log:
>>
>> [snip...]
>> (II) xfree86: Adding drm device (/dev/dri/card1)
>> (EE)
>> (EE) Backtrace:
>> (EE) 0: /usr/bin/X (xorg_backtrace+0x48) [0x7f7e3c6f5fc8]
>> (EE) 1: /usr/bin/X (0x7f7e3c555000+0x1a4ff9) [0x7f7e3c6f9ff9]
>> (EE) 2: /lib/x86_64-linux-gnu/libc.so.6 (0x7f7e3a6d5000+0x36cb0)
>> [0x7f7e3a70bcb0]
>> (EE) 3: /usr/bin/X (0x7f7e3c555000+0xb3539) [0x7f7e3c608539]
>> (EE) 4: /usr/bin/X (xf86BusProbe+0x9) [0x7f7e3c5dc1e9]
>> (EE) 5: /usr/bin/X (InitOutput+0x728) [0x7f7e3c5ea668]
>> (EE) 6: /usr/bin/X (0x7f7e3c555000+0x57dab) [0x7f7e3c5acdab]
>> (EE) 7: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xf5)
>> [0x7f7e3a6f6f45]
>> (EE) 8: /usr/bin/X (0x7f7e3c555000+0x4352e) [0x7f7e3c59852e]
>> (EE)
>> (EE) Segmentation fault at address 0x0
>> (EE)
>> Fatal server error:
>> (EE) Caught signal 11 (Segmentation fault). Server aborting
>> (EE)
>>
>>  <snip>
>>
>> Can you give me any comments regarding these two problems?
> It looks like that crash is happening way before any driver gets
> loaded, so I'm not really sure what's going on there. I'd need to know
> exactly how X was getting invoked by startx and which OS's build of the
> X server you were using to figure out more.
>
> startx isn't really the tool you want to use to debug just starting the
> X server though, it tries to bring up a session as well. If you can ssh
> in from another machine and start X directly from gdb your debugging
> life will be much more pleasant.

Just for reference:

Running OS: Ubuntu 15.04
X Server: 1.17.1
X Built Operating System: Linux 3.19.0-28-generic X86_64 Ubuntu
Running Kernel: 3.18.29

>
> - ajax

Best regards,
Jose Miguel Abreu