On 12/18/2012 07:21 AM, Rob Clark wrote:
On Mon, Dec 17, 2012 at 11:04 PM, Dave Airlie<airlied@xxxxxxxxx> wrote:
So this might be a bit off topic but this whole CDF triggered me
looking at stuff I generally avoid:
The biggest problem I'm having currently with the whole ARM graphics
and output world is the proliferation of platform drivers for every
little thing. The whole ordering of operations with respect to things
like suspend/resume or dynamic power management is going to be a real
nightmare if there are dependencies between the drivers. How do you
enforce ordering of s/r operations between all the various components?
There have been already some ideas proposed to resolve this at the PM
subsystem level [1]. And this problem is of course not only specific
to platform drivers. The idea of having monolithic drivers, just because
we can't get the suspend/resume sequences right otherwise, doesn't really
sound appealing. SoC IPs get reused on multiple different SoC series,
no only by single manufacturer. Whole graphics/video subsystems are
composed from smaller blocks in SoCs, with various number of distinct
sub-blocks and same sub-blocks repeated different number of times in
a specific SoC revision.
Expressing an IP as a platform device seems justified to me, often these
platform devices have enough differences to treat them as such. E.g.
belong in different power domain/use different clocks. Except there is
big issue with the power management... However probably more important
is to be able to have driver for a specific IP in a separate module.
And this suspend/resume ordering issue is not only about the platform
devices. E.g. camera subsystem can be composed of an image sensor
sub-device driver, which is most often an I2C client driver, and of
multiple SoC processing blocks. The image sensor can have dependencies
on the SoC sub-blocks. So even if we created monolithic driver for the
SoC part, there are still two pieces to get s/r ordering right - I2C
client and SoC drivers. And please don't propose to merge the sensor
sub-device driver too. There has been a lot of effort in V4L2 to
separate those various functional blocks into sub-devices, so they can
be freely reused, without reimplementing same functionality in each
driver. BTW, there has been a nice talk about these topics at ELCE [2],
particularly slide 22 is interesting.
I believe the solution for these issues really needs to be sought in the
PM subsystem itself.
I tend to think that sub-devices are useful just to have a way to
probe hw which may or may not be there, since on ARM we often don't
have any alternative.. but beyond that, suspend/resume, and other
life-cycle aspects, they should really be treated as all one device.
Especially to avoid undefined suspend/resume ordering.
[1] https://lkml.org/lkml/2009/9/9/373
[2]
http://elinux.org/images/9/90/ELCE2012-Modular-Graphics-on-Embedded-ARM.pdf
Thanks,
Sylwester
--
To unsubscribe from this list: send the line "unsubscribe linux-media" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
