[Intel-gfx] DRM Inquiry

Fri May 25 15:56:44 UTC 2018

Looks like the seek=%d in the sprintf is not working. 0x11 0x0A are being returned by the monitor from DPCD’s 0x0000 and 0x0001 repeatedly. The first is DPCD revision (1.1) and the second is maximum Link Rate (0x0a) which is 2.7 Gbps. You might want to do a printf of call to make sure seek is being set correctly.

Which brings up another issue: eDP Backlight Brightness LSB is at hex 0x723 or 1827 decimal. You might also want to confirm your panel supports DPCD backlight adjustment by reading DPCD 0x701 and confirm bit 0 is set.

Clint

From: Intel-gfx <intel-gfx-bounces at lists.freedesktop.org> On Behalf Of John Sledge
Sent: Friday, May 25, 2018 1:11 AM
To: dri-devel at lists.freedesktop.org; Jani Nikula <jani.nikula at linux.intel.com>
Cc: intel-gfx at lists.freedesktop.org
Subject: Re: [Intel-gfx] DRM Inquiry

Hi Jani,

I seek 0-800 and here's what I get, all 11 0A in hex. Not sure if this is the brightness value of the display. I also did a test, when I disconnect the DP to the display and execute the dd commands, it would say error reading 'dev/drm_dp_aux1': Connection timed out. So I think my display setup is okay and the 11 0A values are really coming out from the display.

Output in hex:
11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A 11 0A ....

int main(int argc, char **argv)
{
int ret = 0;
int offset = 0;
char call[100];
for(offset=0;offset<800;offset++)
{
sprintf(call,"dd if=/dev/drm_dp_aux1 bs=1 count=2 seek=%d >> out.txt",offset);
ret = system(call);
}
    return 0;
}
Regards,
John

On Friday, May 25, 2018, 2:56:04 PM GMT+8, Jani Nikula <jani.nikula at linux.intel.com<mailto:jani.nikula at linux.intel.com>> wrote:

On Fri, 25 May 2018, John Sledge <john_sledget at yahoo.com<mailto:john_sledget at yahoo.com>> wrote:
>  Hi Jani,
> I can now see /dev/drm_dp_aux*.
> I'm not familiar with dd command.Correct me if I'm wrong.
> Possible commands i tried and nothing happen:dd if=/dev/drm_dp_aux1 seek=723 ibs=2dd of=/dev/drm_dp_aux1 seek=723 ibs=2
> dd if=/dev/drm_dp_aux2 seek=723 ibs=2dd of=/dev/drm_dp_aux2 seek=723 ibs=2
> I assumed I could read the brightness msb and lsb using the define in drm_dp_helper.h.
> #define DP_EDP_BACKLIGHT_BRIGHTNESS_MSB     0x722#define DP_EDP_BACKLIGHT_BRIGHTNESS_LSB     0x723

You're mixing hex and decimal numbers, you should probably use bs=1
count=2, dd outputs binary so you probably need to pipe it to hexdump to
see anything, etc. Perhaps start experiments with reading at seek=0.

> From here, I was thinking if I could try to open \dev\drm_dp_aux* then read the brightness offset 0x723, though not sure how to proceed with it.I was able to successfully open \dev\drm_dp_aux1 and \dev\drm_dp_aux2 but I thinking I'm wrong when I proceed to ioctl because they all failed.

It's a character device, open, seek, read/write. Don't try any ioctls on
it.

Good luck.

BR,

Jani.

>
> #define BRIGHTNESS 0x723
> int main(int argc, char ** argv){  int fd;  int retcode;  char out[128];    if((fd = open("/dev/drm_dp_aux1",O_RDWR)) >=0)  {    printf("open success");  }  else  {    printf("open failed");
>   }
>    if((retcode = ioctl(fd,BRIGHTNESS,&out)) < 0)  {    printf("ioctl failed");  }  else  {    printf("ioctl success");
>   }
>   // trying aux2  if((fd = open("/dev/drm_dp_aux2",O_RDWR)) >=0)  {    printf("open success");  }  else  {    printf("open failed");
>   }
>   if((retcode = ioctl(fd,BRIGHTNESS,&out)) < 0)  {    printf("ioctl failed");  }  else  {    printf("ioctl success");
>   }
>   return 0;}
> Thanks,John
>
>    On Thursday, May 24, 2018, 8:38:02 PM GMT+8, Jani Nikula <jani.nikula at linux.intel.com<mailto:jani.nikula at linux.intel.com>> wrote:
>
>  On Thu, 24 May 2018, John Sledge <john_sledget at yahoo.com<mailto:john_sledget at yahoo.com>> wrote:
>> I was able to update my kernel to 4.6 which has the DRM_DP_AUX_CHARDEV
>> in the Kconfig file linux-4.6\drivers\gpu\drm. Though I also
>> add DRM_DP_AUX_CHARDEV=y in  kernel config. When invoke uname -r, I
>> could see that the kernel is now 4.6.
>
> If you're updating kernels, why not update to a recent kernel that's
> actually supported...?
>
>> How can I verify the DRM_DP_AUX_CHARDEV takes effect or got configure
>> it correctly?
>
> Boot the kernel, run 'ls /dev/drm_dp_aux*'. If you see stuff, you got it
> right.
>
>> It still unclear to me how to follow what you mean by using DRM DP AUX
>> interface and getting /dev/drm_dp_auxN node(s) that allows me to read
>> and write arbitrary DPCD offsets.
>
> The device is a char device you can open, seek to an offset (which would
> be the DPCD offset), and read. For testing, you can achieve the same
> using dd.
>
> BR,
> Jani.

--
Jani Nikula, Intel Open Source Graphics Center
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