Hi Grant,
On Thu, Nov 15, 2012 at 06:03:59PM +0000, Grant Likely wrote:
On Thu, 15 Nov 2012 17:00:57 +0100, Laurent Pinchart laurent.pinchart@ideasonboard.com wrote:
Hi Grant,
On Thursday 15 November 2012 15:47:53 Grant Likely wrote:
On Thu, 15 Nov 2012 10:23:52 +0100, Steffen Trumtrar wrote:
Add display_timing structure and the according helper functions. This allows the description of a display via its supported timing parameters.
Every timing parameter can be specified as a single value or a range <min typ max>.
Also, add helper functions to convert from display timings to a generic videomode structure. This videomode can then be converted to the corresponding subsystem mode representation (e.g. fb_videomode).
Signed-off-by: Steffen Trumtrar s.trumtrar@pengutronix.de
Hmmm... here's my thoughts as an outside reviewer. Correct me if I'm making an incorrect assumption.
It looks to me that the purpose of this entire series is to decode video timings from the device tree and (eventually) provide the data in the form 'struct videomode'. Correct?
For the time being it is straight from devicetree via struct videomode to struct drm_display_mode or fb_videomode. Correct.
If so, then it looks over engineered. Creating new infrastructure to allocate, maintain, and free a new 'struct display_timings' doesn't make any sense when it is an intermediary data format that will never be used by drivers.
Can the DT parsing code instead return a table of struct videomode?
See below.
But, wait... struct videomode is also a new structure. So it looks like this series creates two new intermediary data structures; display_timings and videomode. And at least as far as I can see in this series struct fb_videomode is the only user.
struct drm_display_mode is also a user in this series see 5/6 and 6/6.
struct videomode is supposed to slowly replace the various video mode structures we currently have in the kernel (struct drm_mode_modeinfo, struct fb_videomode and struct v4l2_bt_timings), at least where possible (userspace APIs can't be broken). This will make it possible to reuse code across the DRM, FB and V4L2 subsystems, such as the EDID parser or HDMI encoder drivers. This rationale might not be clearly explained in the commit message, but having a shared video mode structure is pretty important.
That.
Okay that make sense. What about struct display_timings?
The reason for defining an intermediary step is because of the different things that are described: - struct display_timing describes the signal ranges a display supports - struct display_timings describes all timing settings of a display - struct videomode describes one single mode generated from that settings
It is possible to generate multiple struct videomodes from one struct display_timing based on the circumstances. And that is a task for the driver using the display_timing infos. This means drivers are supposed to use struct display_timings if they need to generate a struct videomode from the timing ranges of one entry. This is just the first step in that direction. I hope this makes the need for struct display_timings a little clearer. The other solution would be the one Laurent suggested and pass multiple values around. Which in my opinion doesn't make it better, more practical or cleaner.
Regards, Steffen